John Dalton (born: September 6, 1766 / died: July 27, 1844)
 
Joseph Dalton was a British chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland.   John received his early education from his father and from John Fletcher, teacher of the Quaker school at Eaglesfield. 

During his residence in Kendal, Dalton had contributed solutions of problems and questions on various subjects to the Gentlemen's and Ladies' Diaries, and in 1787 he began to keep a meteorological diary in which during the succeeding fifteen years he entered more than 200,000 observations. His first separate publication was Meteorological Observations and Essays (1793), which contained the germs of several of his later discoveries; but in spite of the originality of its matter, the book met with only a limited sale. 

In 1794 he was elected a member of the Manchester Literary and Philosophical Society, and a few weeks after election he communicated his first paper on Extraordinary facts relating to the vision of colours, in which he gave the earliest account of the optical peculiarity known as Daltonism or colour-blindness, and summed up its characteristics as observed in himself and others (including his brother). 

Besides the blue and purple of the spectrum he was able to recognize only one colour, yellow, or, as he says in his paper, that part of the image which others call red appears to me little more than a shade or defect of light; after that the orange, yellow and green seem one colour which descends pretty uniformly from an intense to a rare yellow, making what I should call different shades of yellow. This paper was followed by many others on diverse topics on rain and dew and the origin of springs, on heat, the colour of the sky, steam, the auxiliary verbs and participles of the English language and the reflection and refraction of light. 
In 1800 he became a secretary of the society, and in the following year he presented the important paper or series of papers, entitled Experimental Essays on the Constitution of Mixed Gases; on the force of steam or vapour of water and other liquids at different temperatures, both in Torricellian vacuum and in air; on evaporation; and on the expansion of gases by heat. The second of these essays opens with the striking remark,

There can scarcely be a doubt entertained respecting the reducibility of all elastic fluids of whatever kind, into liquids; and we ought not to despair of effecting it in low temperatures and by strong pressures exerted upon the unmixed gases further.
After describing experiments to ascertain the tension of aqueous vapour at different points between 32° and 212° F, he concludes from observations on the vapour of six different liquids, that the variation of the force of vapour from all liquids is the same for the same variation of temperature, reckoning from vapour of any given force. In the fourth essay he remarks, 
I see no sufficient reason why we may not conclude that all elastic fluids under the same pressure expand equally by heat and that for any given expansion of mercury, the corresponding expansion of air is proportionally something less, the higher the temperature. It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from elastic fluids than from other substances
He thus enunciated the law of the expansion of gases, stated some months later by Gay-Lussac. In the two or three years following the reading of these essays, he published several papers on similar topics, that on the Absorption of gases by water and other liquids (1803), containing his Law of partial pressures. 
But the most important of all Dalton's investigations are those concerned with the Atomic Theory in chemistry, with which his name is inseparably associated. The idea of atomic structure arose in his mind as a purely physical conception, forced upon 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 already mentioned, which was read on October 21, 1803 though not published till 1805. Here 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. 
He then proceeds to give what has been quoted as his first table of atomic weights, but on p. 248 of his laboratory notebooks for 1802-1804, under the date 6th of September 1803, there is an earlier one in which he sets forth the relative weights of the ultimate atoms of a number of substances, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time. 

It appears, then, that confronted with the problem of ascertaining the relative diameter of the particles of which, he was convinced, all gases were made, he had recourse to the results of chemical analysis. Assisted by the assumption that combination always takes place in the simplest possible way, he thus arrived at the idea that chemical combination takes place between particles of different weights, and this it was which differentiated his theory from the historic speculations of the Greeks. 

The extension of this idea to substances in general necessarily led him to the law of combination in multiple proportions.  It may be noted that in a paper on the; Proportion of the Gases or Elastic Fluids Constituting the Atmosphere,  the law of multiple proportions appears to be anticipated in the words;

The elements of oxygen may combine with a certain portion of nitrous gas or with twice that portion, but with no intermediate quantity, but there is reason to suspect that this sentence was added some time after the reading of the paper, which was not published till 1805. 
Altogether Dalton contributed 116 memoirs to the Manchester Literary and Philosophical Society of these the earlier are the most important.   In one of them, read in 1814, he explains the principles of volumetric analysis, in which he was one of the earliest workers.

      As an investigator, Dalton was content with rough and inaccurate instruments, though better ones were readily attainable. Sir Humphry Davy described him as a very coarse experimenter, who almost always found the results he required, trusting to his head rather than his hands. Dalton says he had so often been misled by taking for granted the results of others that he determined to 'write as little as possible but what I can attest by my own experience', but this independence he carried so far that it sometimes resembled lack of receptivity. Thus he distrusted, and probably never fully accepted, Gay-Lussac's conclusions as to the combining volumes of gases.
He held peculiar and quite unfounded views about chlorine, even after its elementary character had been settled by Davy. He persisted in using the atomic weights he himself had adopted, even when they had been superseded by the more accurate determinations of other chemists and he always objected to the chemical notation devised by J. J. Berzelius, although by common consent it was much simpler and more convenient than his cumbersome system of circular symbols. His library, he was once heard to declare, he could carry on his back, yet he had not read half the books it contained.

Dalton had requested that his eyes be examined after his death, in an attempt to discover the cause of his colour-blindness; he had hypothesised that his aqueous humour might be coloured blue. Postmortem examination showed that the humours of the eye were perfectly normal. However, an eye was preserved at the Royal Institution, and a 1990s study on DNA extracted from the eye showed that he had lacked the pigment that gives sensitivity to green; the classic condition known as a deuteranope.