On certain Molecular Actions of Crystalline Particles, S?c. 95 



upon by the atmosphere than the phosphate of ammonia. 

 The magnesian solution was generally a few grains of sulphate 

 of magnesia to the same quantity of water as above. 



A small quantity of the first mixture is poured on a piece 

 of glass, and to this are added a few drops of the magnesia in 

 solution; if it be allowed to remain undisturbed, in a few mi- 

 nutes the surface of the liquid becomes covered with a thin 

 film, and on the glass appear minute shining crystals; but if 

 before these crystals have time to form, any solid substance, 

 as a glass rod or an empty pen, for instance, is passed over 

 the glass through the liquid, the course it follows becomes 

 visible shortly after. The images which are thus formed are 

 double, and may be termed the upper and lower images. 



I will first describe the upper images : — They appear on the 

 surface of the liquid itself, when the film would otherwise have 

 been formed. They are seen immediately after the passage 

 of the pen through the liquid, whereas the lower ones only 

 become apparent a few moments after. Being formed on a 

 moveable surface, they are not perfect representations of the 

 traces that have been made, and are changed and distorted 

 by any movement of the liquid. When the solution of the 

 salt is weak, they frequently disappear a few moments after 

 their formation and are redissolved in the liquid; when the 

 liquid is more concentrated, they likewise disappear, owing 

 to the formation of the film on the surface. The production 

 of these images appears to be independent of the chemical na- 

 ture of the body used for tracing. They may be obtained in- 

 dependently of the lower ones, by drawing a thread gently 

 over the surface of the liquid, without its coming in contact 

 with the surface of the glass. 



The lower images are formed on the surface of the glass, 

 under the upper ones. A few seconds after the tracing has 

 been made upon the glass, they begin to appear, and gra- 

 dually become more distinct. The space of time which 

 elapses before their appearance depends upon the strength of 

 the solution. When it is strong they appear quickly, and when 

 weak they take several minutes before they are visible. 



To cause the formation of any images, the tracing must al« 

 ways be made after the mixture of the two solutions ; under 

 no other circumstances have I been able to create them. 

 Thus, when the tracing is made on a perfectly dry glass, or on 

 one slightly wet, and then immediately covered with the solu- 

 tion, no images will be created. This is likewise the case when 

 we make traces in either the magnesian or the phosphate so- 

 lution before their mixture together. 



The passage of any solid substance in the proper solution 



