OF MICROSCOPIC OBJECTS. 115 



over the slide and crystals of a circular form will begin to 

 appear, and may be modified by the atmosphere in which 

 they are allowed to grow ; thus a moist atmosphere or the 

 reverse, an atmosphere of vapour of ammonia, spirit, benzole, 

 or sulphureous fumes, will each produce a different result, 

 and the modifications thus produced will afford food for very 

 serious reflection on the changes one salt may be made to 

 assume in contact with other agents. These crystals are 

 best mounted in castor oil balsam that is very liquid 

 not balsam in benzole, as the benzole changes the character 

 of the crystal. 



Many new forms may be procured by uniting two totally 

 different salts in solution in certain proportions. This is a 

 field affording new facts and beauties ; but requires some 

 chemical knowledge and much perseverance to obtain very 

 valuable results. One of the most beautiful I have met 

 with has been composed of sulphate of copper and sulphate 

 of magnesia. The flower-like forms and uniformity of 

 crystallization when successful make it well worth a few 

 failures at first ; and as I became acquainted with some new 

 facts in my frequent trials, I will give the preparation of the 

 double salt from the beginning. 



Make a saturated solution of the two sulphates, combined 

 in the proportion of three parts copper to one part mag- 

 nesia, and then add to the solution one-tenth of pure water. 

 No dust or other impurities should have access to the slide, 

 and it should be freed from all traces of grease by cleaning 

 immediately before use with liquor potassse or ammonias. A 

 drop of the solution should then be placed upon it, and by 

 a thin glass rod spread evenly upon the surface. Heat 

 this whilst in a horizontal position until the salt remains as 

 a viscous transparent substance, which will not be effected 

 until it is raised to a high degree. The slide may now be 

 allowed to cool, and when this is accomplished, the flower- 

 like crystals will be perceived forming here and there upon 

 the plate. When these are at any stage in which it is 

 wished to preserve them, a few seconds' exposure to the fire, 

 T 2 



