Septembek 1, 1911] 



SCIENCE 



261 



power, or potential, of a given substance 

 (say niter) at fixed temperature and 

 pressure is not affected by bringing it to- 

 gether with certain other substances (sul- 

 phur, charcoal) the resultant body is to 

 be classed as a mechanical mixture with 

 the substance in question (niter) as one 

 of the constituents. In the simple case of 

 gunpowder it is sufficient to find whether 

 or no the solubility of the niter, in water, 

 for instance, remains unchanged ; but in 

 the most complicated cases the principle 

 remains the same. The condition of com- 

 parison at constant pressure (comparison 

 at constant volume is impossible with sol- 

 ids and liquids) excludes air and such-like 

 "mixtures of gases"; while whenever (as 

 with arsenious sulphide and water) the 

 change in chemical power is so slight as to 

 remain in dispute, a discussion may arise 

 as to whether the resultant "pseudo-so- 

 lution" is to be classed with the solutions 

 or with the mechanical mixtures — quite 

 naturally, for at this point the two classes 

 run into one another. 



All this is simple and has proved im- 

 portant in practise; but being based on a 

 view of solutions radically different from 

 that of the texts under discussion, it can 

 find no place in them. 



The chemical compounds, or rather the 

 substances so classed in the high school 

 texts, may be grouped under three heads. 

 First, silver chloride and other phases of 

 invariable composition, which could be 

 defined by adopting the "law" of com- 

 bination in definite proportions as the defi- 

 nition of the group ; second, water and sim- 

 ilar bodies, which though not phases of in- 

 variable composition, can be "purified" 

 by fractionation without paying special 

 regard to the pressure at which the distil- 

 lation is carried out; and third, a gi'oup 

 of which sulphuric acid may be taken as 

 the type. In the text books, the "chem- 



ical compound" sulphuric acid is de- 

 scribed as "an oily liquid of s.g. 1.838 at 

 15° C"; it is in fact one of the continu- 

 ous series of sulphuric acids — from dilute 

 to fuming — used in the laboratory, and is 

 thus not a phase of invariable composi- 

 tion; neither can it be "purified" by 

 fractionation like water, while to include 

 it among the chemical compounds because 

 it freezes to a homogeneous solid of the 

 same composition, would open the door 

 wider than is consistent with general 

 usage. 



As a matter of fact, the name sulphuric 

 acid and the formula HoSO^ were both 

 introduced into chemistry without any 

 special reference to the properties of this 

 particular liquid, and would in all prob- 

 ability have won their way even if no sub- 

 stance of the composition HoSO^ could be 

 prepared — such, at all events, was the case 

 with the analogous "compounds" car- 

 bonic acid and ammonium hydrate. For- 

 mulas like H,SO„ HXOs, NH.OH and the 

 names that go M'ith them, are merely relics 

 of one of the past attempts to represent 

 symbolically the properties of sohitions; 

 in the old days, reagent bottles of sul- 

 phuric acid, whether concentrated or di- 

 lute, were labelled SO3, then HgSO^ was 

 substituted, and now, perhaps, 211 + SO4, 

 the symbol H0SO4 being retained with a 

 different meaning. The change from one 

 of these systems of formulation to another 

 was due to a study of the properties of 

 solutions as a class; can the text-books on 

 their principles make this clear? Let us 

 see how they deal with symbols. 



The symbols HCl, AgNO,, HNO3 and 

 AgCl are defined to give the compositions 

 and, when known, the vapor densities of 

 the compounds they represent. Follow- 

 ing these definitions the symbol 



HCl + AgNOa = HNO3 -I- AgCl 

 purports to record what happens when 



