302 report— 1859. 



the means of throwing some light upon the question, whether such a thing as 

 single elective affinity does take place, or whether all combinations and de- 

 compositions are not cases of double decomposition? If the action of HC1 upon 

 CuO, N0 5 , or of N0 6 HO upon CuCl, be a case of double decomposition, 

 water must take part in it, and, like all other bodies, must act in proportion 

 to its mass. The proportion of water must consequently be taken into ac- 

 count, as well as those of salt and acid ; and if any means can be devised for 

 ascertaining the amount of decomposition, experiments should be made with 

 solutions of different strengths as well as with the saturated solutions. 



4. The fourth series may be considered as a continuation of the last, and 

 will consist of experiments upon the influence which different proportions of 

 the soluble bases, KO, NaO, BaO, SrO, CaO, and NH t O, exert at different 

 temperatures upon the solubility of one another's salts. Here too we may 

 assume double decomposition ; in fact, all these bodies are present in solu- 

 tion apparently as hydrated oxides. Ammonia exerts a singular influence 

 upon the solubility of some salts, such as sulphate of potash, which it preci- 

 pitates from a strong solution. It would be interesting to include in the 

 inquiry the action of the compound ammonias, as they will no doubt be found, 

 while resembling ammonia in many respects, as regards their chemical func- 

 tion, to differ in respect to their influence on solubility. The action of the 

 compound ammonias is interesting from another point of view likewise. 

 Urea, as is well known, if added to a solution of common salt, causes it to 

 crystallize in octahedrons. If we look upon urea as a diamide in which part 

 of the hydrogen is substituted by a polyatomic radical, all other substances 

 belonging to the same type may perhaps produce a like change. Difference 

 of form in the same substance is generally, perhaps always, accompanied by 

 a change of solubility ; such a difference may perhaps exist between the two 

 forms of common salt; so that this series of experiments is of considerable 

 importance. The crystallization of common salt from urine in the form of 

 octahedrons appears to have been known to Rome de Lisle. Fourcroy and 

 Vauquelin showed* that this change of form was due to urea. Beyond this 

 very important fact, little, if anything, was known of the influence of foreign 

 substances upon the crystalline form of bodies, until the publication of Le- 

 blanc's memoir in 1788f. He made many valuable and interesting ob- 

 servations. But it is to Beudant J, who brought together everything that was 

 previously known on the subject, confirmed and very greatly extended the 

 observations, that the subject is indebted for the interest and importance which 

 it now has, and which has led to many new investigations. Among those 

 who have more recently studied the subject, may be mentioned Nickles, 

 Senarmont, and Pasteur. The latter gives a very remarkable instance of this 

 kind of influence §. The acid malate of ammonia crystallizes in rectangular 

 tables of the rhombic system, sometimes having two parallel edges bevelled ; 



P 



the faces — are never observed whenever the salt is crystallized from a pure 



solution. But if a part be heated until it has begun to be decomposed, and 

 then dissolved in water, and the impure solution allowed to cool, the crystals 

 formed in it have hemiheilral faces, which disappear when the crystals are 

 again made to grow in a pure solution. The action of nitric acid upon the 

 nitrates of potash and of baryta, and of ammonia upon a solution of sulphate 

 of potash, may be due to a change of this kind. The subject has never been 



* Ann. de Chim. et de Phys. vol. xxxii. p. 80 (1799). f Journal de Physique, xxxiii. 

 J Ann. des Mines, iii. (1819), and Traite de Mineralogie. 

 § Ann. de Chim. et de Phys. 3 me Serie, vol. xlix. p. 5. 



