Chemistry. 329 



and especially for the analysis of the clay-band and black- 

 band ironstone of this country. He was led to the appli- 

 cation of these salts, in the course of some recent investiga- 

 tions on the materials and products of the manufacture of 

 alum from ** alum-shale," in which he was much retarded 

 by the want of a ready method for estimating the oxides of 

 iron. The chromates of potash give very exact results, and 

 possess the great advantage that a much larger quantity 

 of material may be operated on than can be conveniently 

 treated by the usual methods. For practical purposes, the 

 bichromate is to be preferred. The process requires no other 

 apparatus than that commonly used for centigrade testing, 

 which is familiar to all persons engaged in chemical pursuits. 

 It may be easily and rapidly executed, occupying only a frac- 

 tion of the time required for the process of estimating iron 

 by precipitation as the sesquioxide ; and it is not interfered 

 with by the presence of alumina and phosp'hates which usually 

 exist in the ore. The method is based on the well-known 

 reciprocal action of chromic acid and protoxide of iron, 

 whereby a transference of oxygen takes place, the protoxide 

 of iron becoming converted into sesquioxide and the chromic 

 acid into sesquioxide of chromium. 



Mr Hunt then read an elaborate and interesting commu- 

 nication on the present state of our knowledge of the chemi- 

 cal action of solar radiations. The paper, which occupied 

 nearly two hours in reading, cannot possibly be given in brief 

 compass. We may state, that it contained a very clear his- 

 torical sketch of all that had been done in the investigation 

 of the action of light in producing chemical changes, and that 

 it entered at great length into the theory of the daguerreo- 

 type, calotype, and other photogenic processes, which are 

 now so much objects of popular interest. 



Dr L. Playfair then read a paper on the condensation of 

 volume in highly hydrated minerals. This paper contained 

 the explanation of a very remarkable law, in virtue of which 

 it appears, that in many solid bodies, which contain water 

 in a state of chemical combination, such a condensation 

 occurs, that they occupy no greater space than the water in 

 them would if frozen into iee. 



