WITH THE MICROSCOPE. 51 



the mixture run freely. The balsam becomes more viscid and 

 gradually gets hard as the chloroform evaporates. Solutions of 

 Canada balsam in chloroform are now much used for mounting 

 insects, tissues, and various objects. 



Mr. Hepworth, of Croft's Bank, was among the first to use a 

 solution of Canada balsam in chloroform for mounting objects. 

 Mr. W. H. Keys (Trans. Mic. Soc., Jan. 1865, p. 19) prepares the 

 solution as follows. Old balsam is mixed with sufficient chloroform 

 to make it quite fluid so that it will drop easily from the lip of the 

 vessel containing it. The prepared balsam is then poured into long 

 thin half-ounce phials, corked up, and set aside for at least a month. 

 The balsam thus prepared is clearer and sets much quicker than it 

 mixed with the chloroform at the time it is required for use. 



9G. Arrangements for pressing down the Thin Glass Cover while 

 the Balsam or Cement is becoming hard Some elastic specimens 

 immersed in Canada balsam, gelatin, and other media require firm 

 pressure to be kept up until the balsam or the cement which attaches 

 them to the slide, hardens. Many substances immersed in fluids may 

 be rendered thinner and more transparent if subjected to moderate, 

 but sustained pressure, while the cement which fixes down the thin 

 glass cover is becoming dry and hard. Others require very firm 

 pressure while the process of drying is proceeding. Several methods 

 have been devised for producing pressure. A very simple plan is to 

 place a small piece of wood, about an inch in height, upon the 

 cover. This may be fixed in its place by passing a piece of 

 thread over it, and tying it at the back of the slide ; or the wood 

 may be kept in its place by a vulcanized India-rubber ring. 

 Ordinary weights may be used, or springs arranged as in 

 the ingenious apparatus devised by Mr. Gorham. My friend 

 Mr. White has also suggested a very simple and effective apparatus 

 for the same purpose. It consists of a bent lever, which, 

 by acting upon a screw, can be forced down upon the thin glass 

 with the amount of pressure required. Another form of instru- 

 ment, with a graduated spring, was designed by the Rev. G. Isbell, 

 pi. XX, fig. 128. The c oinpressorium may also be employed for 

 the same purpose, if a small piece of cork be inserted between 

 the thin glass to which the pressure is to be applied, and the 

 glass of the compressorium itself. 



Mr. Hoblyn, of Bath (Archives of Medicine, vol. Ill, p. 140), 

 has also invented an ingenious arrangement. In this instrument, a 

 number of slides may be placed at the same time, and a graduated 

 pressure exerted upon them, pi. XVI, fig. 96. 



The above pieces of apparatus have however been superseded 



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