320 Dr. T. Sterry Hunt on 



760 millim. being assumed as a unit of volume for all species, 

 the weight of a litre of any other gas or vapour at the standard 

 temperature and pressure is its integral weight. In like 

 manner the integral weight of a liquid species is the weight 

 of the same volume at its boiling-point (as indicated in 1853) 

 under a pressure of 760 millim. ; while for any solid species 

 it is the weight of the same volume at the highest temperature 

 which that species can sustain without undergoing a change 

 of state. In other words, the weight of liquids should be 

 compared with that of the gaseous unit of hydrogen at the 

 temperature at which they are generated by the metamor- 

 phosis, through condensation, of the corresponding gaseous 

 species. In like manner the weight of solids should theoreti- 

 cally be determined at the temperature at which they are 

 generated by the metamorphosis of gaseous or liquid species. 

 The weights thus obtained for equal volumes of the various 

 liquid and solid species, as well as for the gaseous species, are 

 evidently the specific gravities of these species ; that of hy- 

 drogen at the standard temperature and pressure being unity, 

 (H 2 = 2). They are at the same time the integral weights of 

 the species compared. 



5. The advantages of this natural unit of specific gravity 

 over the arbitrary ones hitherto adopted are evident. That 

 which, for motives of convenience, has hitherto been employed 

 for gases and vapours, namely the weight of dry atmospheric 

 air at standard temperature and pressure, is the weight of a 

 mixture of gases and not of a chemical integer : while water, 

 adopted as the unit for liquids and for solids, is, on the con- 

 trary, a liquid integer, condensed by refrigeration from 100° 

 (the temperature of its production at 760 millim. pressure) 

 either to 15° or to 4°, its maximum density. The specific 

 weight of hydrogen at standard temperature and pressure is the 

 truly scientific unit of specific gravity for all bodies, whether 

 gaseous, liquid, or solid; the integral weight of this unit 

 entering as a factor into the specific gravities thus calculated, 

 and causing these to represent the equivalent or integral 

 weights of the species compared. 



6. It is obvious that the integral weight of water must in 

 like manner enter into the specific gravities of liquids and 

 solids for which this species is assumed as unity ; and hence 

 the importance of determining as near as possible its integral 

 weight. The ratio between the weights of equal volumes of 

 water-vapour at 100° and 760 millim., and of water at 0°, as 

 deduced from direct determinations of the density of steam 

 compared with air, and of air compared with water, has been 

 found to be 1 : 1698. If now we attempt to fix this ratio by 



