232 Proceedings of the Royal Irish Academy. 



sensitive to light, the gelatine becoming insoluble under the faintly 

 oxidizing tendency of the chromic salt. This film is also found, however, 

 to be sensitive to other agencies, and it has been this phenomenon 

 that has puzzled photographers. Such a film becomes insoluble by 

 moistiure, heat, and lapse of time. It would aj^pear in fact that light is 

 not absolutely necessary to bring about the change. The experiments 

 detailed will go far to explain all these phases of the phenomenon. 

 We find that moisture means solution, or the mobility of the chemical 

 molecules, which is absolutely necessary to chemical change, and that 

 solution, even at ordinary temperatures, means a partial decomposi- 

 tion, however infinitesimal that decomposition may be. But when 

 we add time to this, there is no limit to put upon the results of this 

 decomposition. "We also see how greatly the partial decomposition of 

 the chromic salt is accelerated by each increment of heat, and therefore 

 we can understand how such a film would be sensitive to temperature. 

 The salts used in these experiments were not those ordinarily found in 

 commerce, because their neutrality is not sufiiciently well marked for the 

 determination of the delicate reactions detailed above. The yellow chro- 

 mate was found particularly unreliable in this respect. It was purified by 

 adding a few drops of caustic potash, and re-crystaUizing twice. The 

 bichromate was purified by adding a few drops of dilute sulphmic 

 acid, crystallizing to a slight extent, and rejecting these fii'st crystals, 

 and then taking off a second and more extensive crop of crystals. 

 These last crystals, on re-crystallization, yielded a salt sufficiently pure 

 for the experiments. Solutions made with the two salts exhibited a 

 marked difference in shade from the commercial crystals. 



Another illustration of the dissociation of the acid molecule is well 

 shown in the uranic nitrate : uranic nitrate is, if evaporated to dryness 

 and submitted to heat, decomposed into pui'e uranic hydrate and 

 a basic nitrate. It would probably therefore be a salt which would 

 easily dissociate in solution, even at a moderate temperatui'e, but at 

 the same time, it affords a colour test of the dissociation of the acid 

 molecule, because the presence of free nitric acid causes in the bright 

 yellow solution of ui'anic nitrate an orange coloration. 



It is stated that a solution of uranic nitrate is decomposed at a 

 moderate heat with the deposition of a precipitate, the composition of 

 which has not been determined. I have not found this to be the case, 

 however, in sealed tubes; for it seems to bear with impunity a tempera- 

 ture which must be close upon 130° C. Therefore we are led to infer 

 that such a decomposition results from the loss of nitric acid. When such 

 a solution is heated under these conditions, it gradually becomes 

 more and more orange. Many other instances may be given of the dis- 

 sociation of the acid molecule, and we need hardly go further than 

 the decompositions resulting from the dissociation, when either the 

 acid or the base is volatile. In the case of carbonic anhydride this 

 dissociation results in the production of basic carbonates, such as 



Carbonate of Magnesium ((MgCOs) jMgOSHjO). 



Carbonate of Zinc (ZnCOg (ZnO)^ 33,0). 



Carbonate of Lead (6PbCO,PbOH,0.) 



