TEST-FLUIDS. 127 



basis of a bone or shell, I must dissolve the calcareous 

 portions by means of a mixture of hydrochloric and 

 nitric acid ; if I wish to get rid of the organic matter 

 in sponges, so as to obtain the mineral portion in a 

 separate state, I can do so by boiling the objects in 

 a solution of caustic potash; if it is desirable to 

 harden animal tissues, this can be done by maceration 

 in strong alcohol, or in a solution of chromic acid, 

 "so dilute as to be of a pale straw colour, which 

 is particularly efficacious in bringing into view the 

 finer ramification of nerves." If, on the other hand, 

 I wish to detect the presence of some particular sub- 

 stance in the object I am examining say of starch 

 granules I apply a solution of iodine in water (i gr. 

 of iodine, 3 grs. of iodide of potassium, i oz. of 

 distilled water), and the starch is turned blue; if 

 albuminous substance is present, the test gives it an 

 intense brown. Acid nitrate of mercury colours 

 albuminous substances red. A solution of caustic 

 potash or soda, by means of its solvent power, is ex- 

 tremely useful in rendering animal and vegetable 

 structures transparent. If you wish to clean any glass 

 slides or covers, and to get rid of the Canada balsam 

 or cement, you can readily do by means of spirits 

 of turpentine. 



I shall conclude this very imperfect sketch of some 

 of the marvels of the microscope by quoting some 

 very valuable words of advice of an eminent micro- 

 scopist, Dr. Lionel S. Beale, F.R.S. : 



" No one engaged in the pursuit of any branch of 

 natural science is more tempted to be led into too 

 hasty generalisation than the microscopic observer. 

 It is his duty, therefore, to avoid drawing inferences 

 until he has accumulated a vast number of facts to 

 support the conclusions at which he has arrived. True 

 generalisations and correct inferences promote the 



