Aqueous Solutions of Acids and Alkalies. 309 



occurs at about 500°, where the hydrate has about 7? atoms 

 of water. 



The measure of the test atom of acetic acid is 86*06 sep- 

 tems. It is therefore much larger than the measure of any 

 of the mineral acids, while its specific gravity is greatly below 

 that of any of them. The peculiarities of acetic acid depend 

 upon these two properties. 



Acetic acid of 979° has a specific gravity of 1*057. The 

 addition of as much water as reduces its strength to 800° in- 

 creases its specific gravity to 1*074428. The addition of as 

 much more water as reduces its strength to 400° brings down 

 its specific gravity to 1*056386, which is nearly the same as 

 the specific gravity of the strongest solution of 979°. These 

 singular properties admit of a simple explanation. 



The density of anhydrous acetic acid being not much 

 greater than that of water, little change in the specific gravity 

 of its solutions is produced by simply replacing a proportion 

 of one constituent by the other; yet, if no condensation oc- 

 curred, the density of all the solutions would no doubt run 

 in regular progression from the density of the strongest acid, 

 1-057, to the density of water, 1*000. But in fact, the spe- 

 cific gravity rises from 1*057 to nearly 1*075, and then de- 

 scends towards 1*000. This irregularity results entirely from 

 the condensation that occurs in the formation of the series of 

 hydrates. So great is this condensation, in proportion to 

 the density of the acid, that many of the hydrates are ren- 

 dered by it not only denser than the protohydrate, but denser 

 than even the anhydrous acid itself. The mean density of 

 acid of 800° is 1*046576 [=(-005822 x 8) + 1]. When we 

 add to this the effect of condensation, which is '027852 

 ( = 3*4S15 x 8), we obtain the actual specific gravity of the 

 solution, 1*074428. The mean density of acid of 500° is 

 1*029110 [= (-005822x5)4-1], which, added to the density 

 occasioned by condensation, '034690 (= 6-94 x 5), gives 

 the real specific gravity of acid of that strength, 1*063800. 

 Hence the laws which regulate the specific gravities of solu- 

 tions rule in the same way over acetic acid that they do over 

 sulphuric acid. The singular rise and fall in the specific gra- 

 vity of solutions of acetic acid are due to the existence of no 

 extraordinary property in that acid, but to the regular opera- 

 tion of a simple universal principle that acts alike upon all 

 substances whatever. 



Dalton's hypothesis, that organic substances on being dis- 

 solved in water increase its bulk only by the amount of 

 water pre-existing in them, or by the oxygen and hydrogen 

 which they contain, is at variance with the properties of an- 



