Atomic Volume and Specific Gravity. 503 



itself is obviously twice the volume of ice, 9*8 x 2 = 19*6, the 

 experimental number being 19*5. But all the other salts in 

 this group refuse to arrange themselves under either of the 

 heads which we have found to explain most of the salts in the 

 previous sections. In an exception of this kind we are en- 

 titled to make an assumption which will in all probability be 

 near the truth, if by means of it we can bring into one uni- 

 form system a whole group of anomalous salts. Sesqui- 

 chromate of potash is of great importance in the history of 

 the chromates, from its frequent occurrence, although hitherto 

 it has been altogether neglected by chemists. Chromic acid 

 is actually able to displace sulphuric acid from sulphate of 

 potash, in order to gratify its love for the potash in the pecu- 

 liar condition of the sesquichromate. In numerous instances 

 of decomposition, as will be pointed out by one of us in an- 

 other paper, this sesquichromate appears. The sesquichro- 

 mate is not formed readily, if indeed it is ever formed, by 

 crystallizing chromate of potash with chromic acid in the 

 proportion of sesquichromate, the result being bichromate of 

 potash and chromate of potash, which crystallize separately. 

 Here then is a remarkable point in the constitution of the 

 chromates, which can only be explained by supposing that 

 sesquichromate of potash contains a double atom of chromate 

 of potash united to one of chromic acid. The decomposition 

 of bichromate of potash by oxide of lead necessarily implies 

 that its atom should also be doubled ; 2KO, SCrOg + CrOg 

 boiled with litharge, gives 2KO, SCrOg 4- PbO, CrOg. 



We have found the volume of KO, CrOg to be 37*1, not 

 33*0, as in the case of sulphate of potash. Karsten obtained 

 the specific gravity 2*640, which gives the volume 37*6; and 

 Thomson states the specific gravity to be 2"612, which gives 

 the volume 38*1 ; the mean of all these experiments is 37*6} 

 which, multiplied by 2, for the reasons already stated, gives as 

 the volume of 2(KO, CrOg), 75-2. The natural volume of 

 chromate of potash, deduced from its analogy to sulphate of pot- 

 ash, would be 11 X 3, or on the double atom II x6=sC)6. Now, 

 the assumption we make to explain this class of salts is, that 

 the double atom of chromate of potash enjoys its anomalous 

 character by adding to its natural volume the volume of ice, 

 thus 66*0 + 9*8 = 75*8, which is not very far from the volunie 

 ascertained by experiment. This assumption of a volume of 

 ice, in addition to other volumes of 11, has been shown to 

 exist in the magnesian sulphates, and therefore its hypothe- 

 tical existence in the chromates is by no means extravagant. 

 Sesquichromate of potash must then be the double chromate 

 of potash united to an equivalent of chromic acid, 75*8+ 19'6 



