MICROSCOPES. 259 



they may be free from any pai tides of an extraneous sub- 

 stance. In experimenting, at the outset adapt the focus of 

 the microscope to one of these glass plates. Then, upon 

 one of the larger plates of glass, spread thinly any precipi- 

 tate or other chemical agent with which you wish to work. 

 Lay the glass upon the port-object, and examine leisurely 

 the character and form of the substance. Then, upon one 

 of the smaller glasses, lay, with a glass rod, a small drop 

 of any acid or other fluid with which you wish to act ; and 

 having spread it on the glass, lower it down nearly upon the 

 other glass, so that the edges of the upper glass shall not 

 reach beyond the under one ; as the upper glass, when once 

 brought in contact, should never be disturbed. The two 

 substances will thus, by pressure, be reduced to one uni- 

 formly thin film ; and the action which takes place may be 

 very accurately investigated. 



For instance, let the first glass receive a very minute 

 portion of the carbonate of copper, and the upper glass a 

 drop of nitric acid ; on contact, the carbonic acid of the 

 carbonate will be seen coming away in globules, whilst the 

 carbonate of copper breaks down and disappears. The 

 field is gradually occupied with a solution of nitrate of cop- 

 per ; and this is seen to crystallize in minute rhombic 

 crystals. Lift up the upper glass, and add a small drop of 

 ammonia, and slip down the glass again ; the crystals of the 

 nitrate disappear ; a new combination takes place, and you 

 see the beautiful foliations of the nitrate of ammonia inter- 

 spersed with groups of the still more beautiful prisms of the 

 deep blue ammoniuret of copper. Or, instead of the am- 

 monia, add muriatic acid ; the nitrate is changed into a grass 

 green solution of the muriate of copper, which crystallizes 

 in bundles of spears shooting in all directions across the 

 field of vision. 



These phenomena may be multiplied to any extent by the 

 use of the numerous chemical agents to be obtained. Iodine 

 presents an endless variety of aspects in combination with 

 different agents. The crystallization of hydriodate of 

 potass is very beautiful, if a drop of the solution is put on 

 a glass. Put on the other glass sulphuric acid. When 

 brought in contact, the acid takes the potass, and forms 

 crystals of the sulphate of potass, and the iodine is de- 

 veloped both in solution, which passes off by evaporation, 

 and in the crystalline form of the substance itself. Again ; 



