483 



The combination of these two liquids is attended with contraction 

 of volume ; that is, the volume occupied by the compound is less 

 than the sum of the volumes occupied by its ingredients when un- 

 combined. By means of a simple formula (assuming 1'8485 to 

 be the specific gravity of strong liquid sulphuric acid), we may 6nd 

 what ought to be the specific gravities of the different strengths in 

 Dr Ure's table, were no contraction to take place. By this table we 

 may find the actual specific gravities of such mixtures ; and dividing 

 the actual or observed specific gravity by the calculated specific gra- 

 vity, and deducting unity from the quotient, we have the propor- 

 tional condensation. 



The proportional condensation is greatest for strength 73 of Dr 

 Ure's table, which is the strength of a hydrate composed of one 

 atom of liquid acid and two atoms of water. 



But it is not necessary to suppose all the strengths of Dr Ure's 

 table to be formed by mixing together strong liquid acid and water ; 

 for, taking a certain strength as our standard, we may suppose all mix- 

 tures stronger than it to be formed by mixing it with strong acid, 

 and all mixtures weaker than it to be formed by mixing it with 

 water in certain proportions. 



On this hypothesis we shall have different calculated specific gra- 

 vities, and consequently, different proportional condensations from 

 those obtained when all strengths were viewed as composed of strong 

 acid and water. 



It was shown that a great range of standard strengths gives a 

 maximum at 73, as before, while others indicate a maximum between 

 84 and 85, denoting a hydrate composed of one atom of liquid acid and 

 one of water. These results were made visible by a curve, of which 

 the abcissae represented strengths, and the ordinates proportional con- 

 densations, and it was shown that points of greatest elevation or de- 

 pression, or more generally peculiarities in the curve, denoted de- 

 finite compounds of acid and water. By means of such a curve the 

 following hydrates may be indicated, in addition to those already men- 

 tioned i—SOjHO + 5H0, SO3HO + 7HO. SO3HO + 8HO, SO3 

 HO 4- IIHO, SO3HO + 12H0, and SO3HO + 15H0. 



Independent experiments were made in order to see how far Dr 

 Ure's observations were reliable ; and a remarkable agreement was 

 found for the weaker strengths tried ; but in the higher strengths the 

 observations seemed to show a constant error in Dr Ure's results. 



