Royal Society. 365 



■with Helix nemoralis, Helix virgata. Helix caperata, and Helix hispida. 

 Helix rotundata burrows into decayed wood to increase the size 

 of its shell. Zonites radiatulus appears to remain on decaying blades 

 of grass ; whilst Pupa umbilicata, Clausilia nigricans and Bulimus 

 obscurus bur}' their heads only to increase their shells. With respect 

 to Zo7iites cellarius, Zonites lucidus, and Zonites nitidulus, it was 

 not satisfactorily ascertained whether their heads were buried du- 

 ring the process of growth. 



Observatory, Beeston, E. J. Lowe. 



1854, February 14th. 



2. "Note on the Decomposition of Sulphuric Acid by Penta- 

 chloride of Phosphorus." By Alexander Williamson, Ph.D., F.C.S., 

 Professor of Practical Chemistry in University College. 



Chemists have long been aware of the fact that some acids unite 

 with bases in one proportion only, others in two or more proportions. 

 Thus a given quantity of nitric acid forms with what is termed its 

 equivalent of potash, a definite nitrate of potash ; if less than this 

 equivalent quantity of potash were added to the nitric acid, the 

 product would be a mechanical mixture of the same nitrate of potash 

 with uncombined nitric acid ; if more than the equivalent of potash 

 were added, the excess of alkali would remain uncombined. Sul- 

 phuric acid, on the other hand, is capable of forming two compounds 

 with potash, and it dej)ends upon the proportions in which the two 

 substances are brought together whether the neutral or acid sulphate 

 is formed. 



The number of compounds which an acid forms with one base is 

 now considered as indicating its atomic weight. The weights of 

 sulphuric and nitric acids which are respectively susceptible of neu- 

 tralizing the same quantity of potash are termed equivalent, but 

 these are by no means the same as their atomic weights. Sixty- 

 three parts of nitric acid (nitrate of water) contains the same quantity 

 of hydrogen as forty- five parts of sulphuric acid, and when they are 

 neutralized by potash the whole of this hydrogen is removed and 

 replaced by potassium ; and if neither of the acids could combine in 

 any other proportion with potash, their atomic weights would be the 

 same as their equivalent weights. But sulphuric acid also forms a 

 potash compound in which half of its hydrogen is replaced by potas- 

 sium, the other half remaining in the compound, whereas the smallest 

 particles of nitric acid either exchange the whole or none of their 

 hydrogen for potassium. 



This fact is expressed in the simplest possible manner by the 

 statement that the smallest indivisible particles of sulphuric acid 

 contain two ato.ns of hydrogen, whilst those of nitric acid only 

 contain one. Thus it is, that whereas the equivalent weights of the 

 two acids are the quantities which contain the same amount of basic 

 hydrogen, their atomic weights must be in the proportion of two 

 equivalents of sulphuric to one of nitric acid. The simplest expres- 

 sion for an atom of nitric acid being empirically NO., II, we shall 

 accordingly represent an atom of sulphuric acid by the formula 

 SOiH^,. In like manner, an atom of common phosphoric acid. 



