52 HOW TO WORK 



liy the use of the simple spring clip devised l>y Dr. Maddox 

 (Trans. Mic. Soc., July, 1865, p. 84). This is made by bending a 

 piece of brass wire in the form represented in pi. XVI, fig. 89. 

 The end which is to press upon the thin glass must be filed per- 

 fectly fiat, or a piece of flat cork may be fixed to it. Or, in cases 

 where the glu.-.s cover is very thin, a smaller piece of thicker 

 glass may be placed upon it and the spring allowed to press 

 upon the latter. This clip has been modified by Mr. Webb, as 

 represented in fig. 90. These clips may be obtained at is. bit. 

 ;>nd 2S. per dozen of Mr. Baker, Holborn, and of Mr. Highley, 

 C.reen Street, Leicester Square. 



O7. Gum. Thick gum-water will be found very useful for attach- 

 ing labels to preparations, and also for fixing on the cover when pre- 

 parations are mounted in the dry way. It is prepared by placing 

 common gum-arabic in cold water, and keeping the bottle in a warm 

 place until the solution has become sufficiently thick. It should 

 always be strained before it is placed in the bottle for use. 



Gum-water, thickened with powdered starch or whiting, is a very 

 useful cement for fixing the glass cover on preparations mounted dry. 

 When dry it forms a hard white coating. The addition of a 

 little arsenious acid will prevent the growth of mildew. Another 

 very convenient solution is made by dissolving powdered gum in 

 a weak solution of acetic acid. 



98. French Cement composed of Lime and India-rubber. The 

 French cement composed of lime and India-rubber is very valuable 

 for mounting all large microscopical preparations. The principal 

 advantages are, that it never becomes perfectly hard, and it therefore 

 permits considerable alteration to take place in the fluid contained in 

 the cell without the entrance of air. It also adheres very intimately 

 to glass, even if it be perfectly smooth and unground. Suppose a 

 glass cover is to be attached to a large cell containing fluid. A small 

 piece of the cement is taken between the finger and thumb am! 

 carefully rolled round until it can be drawn out into a thread about 

 the eighth or tenth of an inch in thickness. I apply this to the top 

 of the cell, before introducing any fluid, and slightly press it down 

 with the finger previously moistened. It adheres intimately. The 

 preservative fluid with the preparation are now introduced and the 

 cell filled with fluid which indeed is allowed to rise up slightly above 

 its walls. The glass cover, cut rather smaller than the external dimen- 

 sions of the cell, and slightly roughened at the edges, is to be gently 

 breathed upon, and then one edge is applied to the cement, so that 

 it maybe allowed to fall gradually upon the surface of the fluid which 

 is now seen to wet each part of the cover successively, until it com- 



