46 OBJECTS AND THEIR MOUNTING. 



flat, then the process described (section 39) must be resorted ; Mr. 

 Goadby's solution being substituted for the spirit of wine. 



41. The marine glue is certainly the best cement for joining 

 pieces of glass together, and it is also very useful in forming the cells 

 for the reception of microscopic objects in lieu of the coatings of 

 white lead, of which we have already spoken, (section 39.) It is 

 made thus one pound of caoutchouc is to be dissolved by maceration 

 for several days in four gallons of coal naptha, and with one pint of 

 this solution, two pounds of shellac are to be mixed by heat When 

 the fusion is complete, it is to be poured out on a cold slate, and 

 moulded into convenient forms for use. When cold, it is as hard as 

 wax. It is applied by gently heating the glass, and then rubbing on 

 the glue. So tenacious is its hold, that the joint will rarely, if ever 

 give way, the glass may be shivered to atoms, but the joints will 

 remain firm as ever. 



42. The following mode of preserving the crystals of salts, as 

 permanent objects for the microscope, and for the exhibition by that 

 instrument of the phenomena of polarized light, is due to the researches 

 of Mr. Warrington. The method to be adopted in mounting the 

 specimens, is as follows : A warm saturated solution of the salt 

 required is to be prepared, and a drop of it placed upon the glass 

 slider on which it is intended to be permanently mounted, and allowed 

 to crystallize ; when a good group of crystals is obtained, the uncrys- 

 tallized portion is to be cautiously removed, this is best effected by 

 drawing it gradually away in a small stream along the edge of the 

 slider : having previously broken through that part of the crystalline 

 ring adjacent to the edge, the salt is to be allowed to drain itself quite 

 dry, by placing the slider on its end in a vertical position. It should 

 next be examined, under the microscope, to ascertain the fitness of the 

 crystals for the purposes required ; because many salts separate from 

 their solutions in crystals too thin to exhibit any prismatic colours 

 when viewed by s polarized light, appearing only of a pearly or silvery 

 aspect, while others form in the opposite extreme, and are totally 

 unfit, from their thickness, for investigation. Presuming, however, 

 that the crystals are such as the investigator requires, the next step 



