

136 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI 



his mind. In this he resembled Cavendish ; but he 

 did not resemble him, and in one sense may be said 

 to have surpassed him, in the boldness, we might call 

 it audacity, with which on a frequently too slender 

 foundation of facts Dalton established, to his own 

 satisfaction, by reasoning almost as much a priori as 

 that of the mathematician, comprehensive physical 

 theorems expressing the laws of phenomena. 

 (612.) The more important of these refer to the consti- 

 Dalton's tution of gases and vapours, together with their rela- 

 writmgs. t j ons to hga^ g^ to t fr e combinations of bodies by 

 chemical affinity. We shall endeavour to give some 

 account of these researches separately. They are to 

 be found detailed in the Memoirs of the Literary and 

 Philosophical Society of Manchester (particularly the 

 fifth volume), and in Dalton's New System of Che- 

 mical Philosophy, of which three successive parts ap- 

 peared in 1808, 1810, and 1827, of which the first 

 is the most original and important. 



(613.) I. We shall first speak of his researches connected 

 Researches W j lt j 1 g ases an( j vapours. It had been noticed by 

 and va- Priestley and others that when gases exercising ap- 

 pours, parently no chemical action upon one another, are 

 mixed in a confined space, they become, after the 

 lapse of a longer or shorter time, completely inter- 

 mingled, and that without any regard to even great 

 differences in their density, the particles of the lighter 

 gas diffusing themselves contrary to gravity through 

 those of the denser, and causing them in their turn to 

 ascend. The uniform composition of our atmo- 

 sphere, in all circumstances and at all heights, is a 

 striking example of this property. In explanation of 

 it, Dalton had in 1801 arrived at the conclusion, 

 " that the particles of one gas are not elastic or re- 

 pulsive in regard to the particles of another gas, but 

 only to the particles of their own kind." The fun- 

 damental experiment on which this singular con- 

 clusion was based was the following: That the 

 quantity of vapour of water (which, when purely 

 elastic, may be considered as a gas) which can exist 

 uncondensed in a given space, depends solely upon 

 the temperature, and is independent of the presence of 

 air and of its own density. Thus water, or any other 

 evaporable liquid, being introduced into a space con- 

 taining air, or any other gas. of any density, but 

 subject to a constant external pressure, the space in 

 question becomes damp by the evaporation of the 

 liquid ; now the amount of the latter converted into 

 vapour depends, according to Dalton, upon the single 

 circumstance that the vapour yielded by it must 

 have the precise elasticity due to its temperature. 



Its elastic force being added to that of the dry air, 

 the whole will expand until equilibrium is restored 

 with the constant pressure without, and this will oc- 

 cur as soon as the elasticity of the dry air alone 

 (proper to its increased volume), added to the elasticity 

 of the vapour alone (depending solely on its tempera- 

 ture), are together equal to the pressure which they 

 have to support. In short, to use the precise enun- 

 ciation of Dalton himself, " in all cases the vapour 

 rises to a certain force, according to temperature, 

 and the air adjusts the equilibrium by expanding or 

 contracting as may be required." 1 



The importance of this law (easily verified in the (614.) 

 particular case) is readily perceived. Not only did Theory o 

 it affect the results of almost every experiment in n yg rome 

 pneumatic chemistry, but it rendered a new theory of 

 hygrometry indispensable. The older theory of Hal- 

 ley, Leroy, and Franklin was, that the direct affinity 

 between air and water drew up a portion of the lat- 

 ter into the former with the aid of heat, whilst De 

 Saussure (Essai sur VHygrom6trie t 1783) believed 

 that the conversion of water into vapour took place 

 first and independently by the action of heat, and that 

 it was then drawn up into the atmosphere by the at- 

 traction of the gases which compose it. Dalton's 

 experiments show that the air and the vapour mix 

 without the slightest mutual interference or reac- 

 tion. He founded thereupon an excellent prac- 

 tical method of determining the amount of vapour 

 in the atmosphere. He first formed a table of the 

 elasticities of watery vapour or steam for all tem- 

 peratures between 32 and 212; and so simple and 

 accurate were his methods, at least for the lower de- 

 grees of the thermometer, that his numbers are still 

 received as amongst the best we have. He operated 

 merely with a carefully constructed barometer, into 

 the vacuum of which he introduced a few drops of 

 water, and raising the temperature by means of a 

 tube embracing it, and which could be filled witli 

 water at pleasure, he observed carefully the depres- 

 sion of the mercurial column. The elasticities thus 

 determined commenced with 0'2 inches of mercury at 

 32 up to 30 inches at 212. These numbers, con- 

 sequently, represent the utmost elasticities of vapour 

 which can exist either in air or without air at the cor- 

 responding temperature. If we attempt to add more 

 vapour, or to lower the temperature, in either case 

 moisture will be deposited. Hence to find the quan- Dew-poin 

 tity of vapour in the atmosphere when not absolutely 

 damp, it is sufficient to ascertain the temperature at 

 which it becomes so. This Dalton did by filling a 



1 The exact expression of the effect is this, w=.-.P , where p represents the pressure expressed in inches of mercury upon a 



P f 



given volume (equal to unity) of dry air ; / the force of the vapour in vacuo at the temperature of experiment, also in inches of 

 mercury ; and v the volume which the mixture of air and vapour occupies under the given pressure p after saturation. It is evident 

 that/) / being the pressure due to the elasticity of the dry air apart from 'the vapour, when we affirm that the volume (1) becomes 



, we in effect affirm that Mariotte's (or Boyle's) law connecting volume and pressure holds true for air which is mixed with 



vapour, just as though vapour were absent or its space void. 



