276 MANIPULATION. 



the two cemented surfaces, and cause their separation in a 

 short space of time. 



Electrical Cement. — A very useful cement for some purposes 

 hereafter to be described, is made by melting together ten 

 ounces of resin and two ounces of bees'-wax, adding two 

 ounces of red ochre and a teaspoonfull of plaster of Paris ; 

 this cement is generally employed for fastening brass or wood 

 to glass in all kinds of electrical apparatus ; it must be used 

 when hot, and can readily be fashioned into any shape before 

 it gets quite cold. Another very excellent but less brittle 

 cement is made by melting together two ounces of black resin, 

 one ounce of bees'-wax, and one ounce of vermilion ; this will 

 be useful for making the thin flat cells described at page 288, 

 as well as for many other purposes. 



Several other cements will be required occasionally by the 

 microscopist, viz., a thick solution of gum arabic in water, to 

 which a small quantity of essential oil has been added, to 

 prevent it from fermenting and becoming sour; also, the 

 same powdered and dissolved in acetic acid or distilled vine- 

 gar ; as a substitute for these, the liquid sold by Messrs. 

 Ackermann as diamond cement will be found of great value, 

 both as a cement and as a fluid for mounting some kinds of 

 objects. Mastich varnish is also very useful for cementing 

 opaque objects to discs of cork or leather, or to any of the other 

 kinds of surfaces on which they are intended to be mounted. 



For covering slides with paper, common paste is a good 

 cement, but as it soon gets mouldy and sour, the following 

 mixture, employed by Mr. Jackson, is a good substitute : — 

 Powdered gum tragacanth, 1 oz. ; powdered gum arabic, 

 2 oz. ; white sugar, 2 oz. ; mixed and kept in a dry state. 

 When wanted, mix up a small quantity with water by means 

 of a brush, called by painters a "Jitch tool" and if what is not 

 used be kept in an open shallow vessel, it will dry before it 

 becomes mouldy, and may readily be rubbed up again with 

 water. 



White-lead, ground in linseed-oil, will be required for 

 making the cells described at page 286 ; this should be old, 



