Molecular Actions of Crystalline Particles, fyc. 97 



stals. Another method of demonstrating the difference of 

 their adherence, is by allowing the solution to dry on the 

 glass, when by brushing it slightly with the feather of a pen, 

 most of the irregular crystals are taken off and the images re- 

 main. 



Other substances capable of forming a like deposit. — Chloride 

 of platinum and nitrate of potash, mixed together, form a 

 double chloride, with which images can be obtained with as 

 much ease as with the double phosphate. The only differ- 

 ence is, that the double chloride precipitates in the shape of 

 octahedrons, &c. Solutions of tartaric acid and nitrate of 

 potash deposit crystals of bitartrate of potash, which are capa- 

 ble of forming upper and lower images with nearly as much 

 facility as the double phosphate. The lower images formed 

 by the bitartrate differ in one respect from those by the phos- 

 phate, for shortly after their formation they appear to lose 

 their adhesion to the glass, and the slightest agitation of the 

 liquid causes them to be detached ; and if a sentence has been 

 written, the curious appearance is presented of fragments of 

 words and letters floating about in confusion. Under the 

 microscope also they differ, fewer parallel lines are perceived, 

 and the crystals are larger and unequal in size. Liquor po- 

 tassse added to a solution of tartaric acid will form images 

 exactly similar to those just mentioned. Caustic soda and 

 tartaric acid produce the same result, but the solution must 

 be much more concentrated. 



Images formed by gaseous bodies. — These traces are formed 

 in the same manner as those which are crystalline, by passing 

 a solid body over a piece of glass covered with a liquid con- 

 taining a gas in solution, when they are immediately perceived 

 by the bubbles which are deposited. On account of the spe- 

 cific gravity of the gas, these images are not very durable, for 

 after a short time the gas which composes them rises to the 

 surface. As a general rule, the ingredients, whose combina- 

 tion causes the formation of the gas, should be added together 

 gently, and so diluted that whatever gas is formed they re- 

 main dissolved in the liquid. I have been surprised to find 

 how much gas n ay be in this way made to remain in solution ; 

 and as most of them appear capable of being dissolved in this 

 unstable manner, traces may be obtained from them all; and 

 I have ascertained by experiment, that such is the case with 

 carbonic, acetic and hydrochloric acids. 



To obtain carbonic acid, I have generally used the subcar- 

 bonate of soda and tartaric acid. Acetate of ammonia was 

 employed to liberate acetic acid, and hydrochloric acid was 

 obtained from common salt and sulphuric acid. A mixture 



