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-hke 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 potassee or ammonie. 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. 
! 12 
