Sep 062017
 

Today is the birthday (1766) of John Dalton FRS, English chemist, physicist, and meteorologist, best known (by people who know about these things) for proposing the basis of modern atomic theory. Dalton was born into a Quaker family in Eaglesfield, near Cockermouth, in the Lake District. His father was a weaver. He received his early education from his father and from Quaker John Fletcher, who ran a private school in the nearby village of Pardshaw Hall. Dalton’s family did not have enough money to support him in school for long, so he began to earn his living at the age of 10 in the service of a wealthy local Quaker, Elihu Robinson. It is said he began teaching at a local school at age 12, and became proficient in Latin at age 14.

When he was 15, Dalton joined his older brother Jonathan in running a Quaker school in Kendal, about 45 miles (72 km) from his home. Around the age of 23, Dalton may have considered studying law or medicine, but his relatives did not encourage him, perhaps because being a Dissenter, he was barred from attending English universities. He acquired most of his scientific knowledge from informal instruction by John Gough, a blind natural philosopher. At the age of 27 he was appointed teacher of mathematics and natural philosophy at the “New College” in Manchester, a dissenting academy. He remained there until the age of 34, when the college’s worsening financial situation led him to resign his post and take up a new career as a private tutor in mathematics and natural philosophy.

Dalton’s early life was strongly influenced by Elihu Robinson, who was a competent meteorologist and instrument maker, and who interested him in problems of mathematics and meteorology. In 1787 at age 21 he began his meteorological diary in which, during the succeeding 57 years, he entered more than 200,000 observations. He rediscovered George Hadley’s theory of global atmospheric circulation (now known as the Hadley cell) around this time. In 1793 Dalton’s first publication, Meteorological Observations and Essays, contained the seeds of several of his later discoveries but despite the originality of his treatment, little attention was paid to them by other scholars. His Elements of English Grammar, was published in 1801.

After leaving the Lake District, Dalton returned annually to spend his holidays studying meteorology and climbing mountains to measure their height. He took measurements of temperature and humidity at various altitudes which he estimated using a barometer. Until the Ordnance Survey published maps for the Lake District in the 1860s, Dalton was one of the few sources of information on altitudes in the region.

In 1794, shortly after his arrival in Manchester, Dalton was elected a member of the Manchester Literary and Philosophical Society, the “Lit & Phil”, and a few weeks later he communicated his first paper on “Extraordinary facts relating to the vision of colours”, in which he postulated that inability in color perception was caused by discoloration of the liquid medium of the eyeball. Both he and his brother were color blind, and so postulated (correctly) that the condition must be hereditary. He was able to recognize only blue, purple, and yellow. At the time (and still to an extent today), being colorblind was a severe handicap to being an analytic chemist.

The most important of all Dalton’s investigations concern atomic theory in chemistry. How he came up with the theory is not fully understood. The theory may have been suggested to him either by researches on ethylene (olefiant gas) and methane (carburetted hydrogen) or by analysis of nitrous oxide (protoxide of azote) and nitrogen dioxide (deutoxide of azote). These investigations may have led him to the idea that chemical combination (the production of definable compounds) consists in the interaction of atoms of definite and characteristic weight. Or the idea of atoms may have arisen in his mind as a purely physical concept, forced on him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper “On the Absorption of Gases by Water and other Liquids” where he says:

Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases. This is the germ of the idea that elements are composed of atoms and that the atoms of different elements have different weights.

The main points of Dalton’s atomic theory are:

Elements are made of extremely small particles called atoms.

 Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

 Atoms cannot be subdivided, created, or destroyed.

 Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

Dalton published his table of relative atomic weights containing six elements, hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. He provided no indication in this paper how he had arrived at these numbers but in his laboratory notebook, dated 6 September 1803, is a list in which he set out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.

Compounds were listed as binary, ternary, quaternary, etc. (molecules composed of two, three, four, etc. atoms) in the New System of Chemical Philosophy depending on the number of atoms a compound had in its simplest, empirical form. Dalton hypothesized the structure of compounds can be represented in whole number ratios. So, one atom of element X combining with one atom of element Y is a binary compound; one atom of element X combining with two elements of Y is a ternary compound, and so on. Many of the first compounds listed in the New System of Chemical Philosophy correspond to modern views, although many others do not.

Dalton used his own symbols to visually represent the atomic structure of compounds (see above). They were depicted in the New System of Chemical Philosophy, where he listed 20 elements and 17 simple molecules.

He always objected to the chemical notation devised by Jöns Jakob Berzelius (the one using letters for elements that we use today), although most thought that it was much simpler and more convenient than his own cumbersome system of circular symbols.

Dalton never married and had only a few close friends. As a Quaker, he lived a modest and unassuming personal life. For the 26 years prior to his death, Dalton lived in a room in the home of the Rev W. Johns, a published botanist, and his wife, in George Street, Manchester. Dalton and Johns died in the same year (1844).

Dalton’s daily round of laboratory work and tutoring in Manchester was broken only by annual excursions to the Lake District and occasional visits to London. In 1822 he paid a short visit to Paris, and attended several of the earlier meetings of the British Association at York, Oxford, Dublin and Bristol.

Dalton suffered a minor stroke in 1837, and a second in 1838 left him with a speech impairment, although he remained able to perform experiments. In May 1844 he had another stroke. On 26th July 1844 he recorded with trembling hand his last meteorological observation. On 27th July 1844, in Manchester, Dalton fell from his bed and was found lifeless by his attendant. He was accorded a civic funeral with full honors. His body lay in state in Manchester Town Hall for four days and more than 40,000 people filed past his coffin. The funeral procession included representatives of the city’s major civic, commercial, and scientific bodies. He was buried in Manchester in Ardwick cemetery which was later converted to a playing field, and all the graves moved.

Dalton was a native of Cumbria but he spent all of his working life as a scientist in Manchester so a Manchester recipe is suitable to celebrate his birthday.  Manchester tart is a perennial favorite that used to be a mainstay of school lunches. It’s a rich mixture of raspberries, raspberry jam, and egg custard baked in a tart shell. Some people add sliced bananas as well.

Manchester Tart

Ingredients

butter, for greasing
500g shortcrust pastry
plain flour, for dusting
200g raspberry jam
3 tbsp plain desiccated coconut
3 tbsp desiccated coconut, toasted in a dry frying pan until golden-brown
300g fresh raspberries
500ml milk
1 vanilla pod, split, seeds scraped out with a knife
5 egg yolks
125g caster sugar
1 tbsp cornflour
2 tbsp icing sugar, for dusting
400ml double cream, whipped until soft peaks form when the whisk is removed

Instructions

Preheat the oven to 200˚C.

Grease a 24 cm tart tin with butter. Roll out the shortcrust pastry on to a lightly floured work surface to a 0.5cm thickness. Line the prepared tart tin with the pastry. Prick the pastry several times with a fork, then chill in the refrigerator for 30 minutes.

Place a sheet of baking parchment into the chilled pastry case and half-fill with dried beans. Transfer the pastry case to the oven and bake for 15 minutes, or until pale golden-brown. Remove the beans and baking parchment and return the pastry case to the oven for a further 4-5 minutes, or until pale golden-brown.

Spread the raspberry jam over the pastry base in an even layer. Sprinkle over the three tablespoons of non-toasted desiccated coconut and half of the fresh raspberries. Set the pastry base aside.

Bring the milk, vanilla pod and vanilla seeds to the boil in a pan, then reduce the heat to a simmer and simmer for 1-2 minutes. Remove the vanilla pod.

In a bowl, beat together the egg yolks and sugar until well combined. Pour the hot milk and vanilla mixture over the egg and sugar mixture, whisking continuously, until the mixture is smooth and well combined. Return the mixture to the pan over a medium heat. Whisk in the cornflour, gradually until well combined, then heat, stirring continuously until the mixture is thick enough to coat the back of a spoon. Transfer the custard mixture to a clean bowl and dust with the icing sugar to prevent a skin forming on the surface of the custard. Set aside to cool, then chill in the refrigerator for 30 minutes.

Fold the whipped double cream into the chilled custard mixture until well combined. Spoon the custard and cream mixture into the pastry case in an even layer. Sprinkle over the remaining fresh raspberries.

To serve, sprinkle over the three tablespoons of toasted desiccated coconut. Serve immediately.