240 



Mr. F. D. Brown on the Physical [May 31, 



stances which differ only in the arrangement o£ their constituent atoms, 

 and not in their number or kind, must, when the tension equals 760 

 millims., occupy the same space, or, in other words, the molecular volumes 

 of isomers at their boiling-points are equal. If this relation exists be- 

 tween the molecular volumes of two isomers at those temperatures at 

 which their respective vapour-tensions are equal to 760 millims., it 

 should equally hold good at all those corresponding temperatures at which 

 the vapour-tensions are equal to any number n of millimetres, since the 

 ordinary barometric pressure of 760 millims. is a special circumstance 

 due to our residence on this planet and to the usual position of our 

 laboratories at no great altitude. 



The study of the vapour-tensions, combined with that of the density 

 and rate of expansion, enables us therefore to compare the molecular 

 volumes at all those corresponding temperatures at which the vapour- 

 tensions are equal ; should a uniform equality be apparent, it is evident 

 that a connexion between vapour-tension and rate of expansion would be 

 established. For, let A and a be two isomeric liquids, D and d their 

 respective densities, and H and h their vapour-tensions at any tempera- 

 ture t, we have D=f(t), d=f'(t), H=0(£), 7i=(j)'(t); if we have 

 observed that whenever H=h D = d*, it of course follows that for any 

 two values t x and t 2 such that 0(^)=0'(^ 2 ) we shall have also /(£,) 



=f(Q- 



Since the expressions (f)(t) and/(£) present no peculiarities in the case 

 of isomeric substances, it would be reasonable to suppose that in the case 

 of other liquids there is also a relation between (f>(t) and f(t). 



A detailed description of the apparatus employed, together with all 

 the precautions taken to ensure accuracy, must be reserved for the future ; 

 a brief statement on this head is, however, indispensable. 



Thermometers. — The two thermometers by which all measurements of 

 temperature were taken are each about 20 inches long, and are divided 

 on the glass into tenths of a degree ; the scale of the one (A) ranges 

 from -7° to 53°, that of the other (B) from 46° to 102°. These instru- 

 ments were most carefully compared with a Kew standard which has 

 long been in the possession of the London Institution. 



The bath in which the comparisons were made was 22 inches in 

 height and 12 inches in breadth, and was filled with water which was 

 rapidly and constantly stirred ; the readings were marked on paper 

 divided carefully into squares, and the points united by a curved line ; 

 the corresponding readings of the air thermometer derived from 

 Begnault's Tables being also marked, the correction for any subse- 



* V and v being the respective molecular volumes, and m the common molecular 

 weight, we have V = — and v —-^'i consequently when V = v, D = d: we may there- 

 fore substitute the equality of the densities for that of the molecular volumes. 



