Cutting, Orinding, and Mounting Hard Tissues 189 



seconds, then carefully washed in alcohol or methylated 

 spirit, gently rubbing the surface with the little finger, finally 

 washing in water and drying with a very soft piece of linen. 



The metal is now examined under the microscope, and it 

 will then be seen if the etching has been sufficient ; if not, 

 it should be repeated, as above, for another twenty seconds. 

 The student should do this several times, noting the effect 

 of the reagent each time until he becomes thoroughly 

 acquainted witli its properties. 



So far we have only dealt with steel, but alloys of tin, 

 copper, etc., are treated in exactly the same way, with the 

 exception that liquorice and iodine are not used. The 

 various acids, ammonia, and caustic potash are then used 

 in weak solution as etching reagents. 



With respect to the mounting of the specimens, it will 

 be seen that the new holder does away entirely with the 

 glass slide. It often happens that the lower edge of the 

 metal is left jagged, and may also be broken off at a very 

 sharp angle, necessitating a long delay in filing or grind- 

 ing, also the metal must not be too thick if a glass slide is 

 used ; but, as will be seen, this labour is greatly minimised 

 if the holder be used. 



LESSON X. 



PREPARING AND MOUNTING ENTOMOLOGI- 

 CAL SPECIMENS FOR THE MICROSCOPE. 

 MR. ROUSSELET'S METHOD OF PRESERV- 

 ING ROTIFERA, ETC. 



Insects should be killed with chloroform. They are then 

 to be placed in methylated spirit, in which they may remain 

 until required for mounting. 



To Prepare a whole Insect for Mounting with 

 Pressure in Canada Balsam. — (1) Transfer from methy- 

 lated spirit to water, and let it soak for three or four hours 

 to remove spirit. 



(2) Place in liq. potassse — 10 per cent, of caustic potash 



