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On the Use of Arabin in Mounting Microscopical Objects. 

 By H. J. Waddington. 



(Read August 27th, 1880.) 



It will have been observed that, when a solution of gum Arabic 

 has been used as an adhesive medium for attaching diatoms or 

 other delicate material to slips, that there is often present a granular 

 appearance, which is owing to the earthy matters contained in the 

 gum. It is impossible to get rid of this by the most careful filtra- 

 tion, these earthy matters being soluble in the same liquids as the 

 gum itself. As there is no adhesive medium so suitable as gum for 

 microscopical purposes, I have thought that a preparation of per- 

 fectly pure gum, free from the objection thus mentioned, would be 

 of some value to microscopists. Such a medium I find in the body 

 known to chemists as Arabin, and which is in reality gum Arabic 

 from which these impurities have been removed. 



To obtain Arabin for microscopical use, gum Arabic should be 

 selected as clear and white as possible. This is to be dissolved in 

 distilled water to the consistence of thin mucilage. It should then 

 be filtered, and the filtrate poured into rectified alcohol, and well 

 shaken. The Arabin immediately separates as a white pasty mass, 

 and the whole becomes semi-solid. It must be placed on filter- 

 paper, and washed with alcohol, until the washings are perfectly free 

 from water, and the alcohol comes off as pure as it went on. The 

 Arabin may now be allowed to dry spontaneously. The edges of 

 the surface of the mass will probably be found to be viscid, owing 

 to the absorption of water from the atmosphere or from the alcohol 

 — but the remainder will be a perfectly pure white powder. This 

 should be shaken off the filter and preseiTed. 



Arabin possesses for microscopical purposes all the properties of 

 ordinary gum. It is freely soluble in water (more readily so than 

 gum), insoluble in alcohol. 



Like ordinary mucilage, it is very liable to become mouldy if kept 

 in solution, but when once dry on a slip it undergoes no change, and 

 shows, at least as far as my own observation is concerned, no dete- 



