196 PREPARATION OF OBJECTS BY CHEMICAL ACTION. 



by removing portions of their shells by the application of diluted 

 Acid, than by grinding down thin sections. The acid (Nitric or 

 Hydrochloric) may be applied with great nicety by means of a fine 

 pointed camel-hair pencil, the object being attached to a slide, and 

 placed under the simple Microscope ; and another camel-hair pencil 

 charged with water should be at hand, to enable the observer to 

 stop the solvent action whenever he may consider that it has been 

 carried far enough. Again, in order to obtain the animal basis of 

 Shell, Bone, Tooth, &c, it is necessary to dissolve away the Cal- 

 careous portion of these tissues by the use of acids ; a mixture of 

 Nitric and Hydrochloric acids is preferable ; and this should be 

 added, little by little, to a considerable bulk of water, until a dis- 

 engagement of gas be perceived to commence from the surface of 

 the specimen. Care should always be taken not to hurry the process 

 by adding too much acid, since, when the animal membrane is of 

 very delicate consistence, it is liable to be dissolved ; and in some 

 cases it is better to allow the action to go on for many weeks, add- 

 ing only a drop or two of acid at a time. When Siliceous particles 

 are to be removed (such as those which form the loricce of the 

 Diatomacea?), for the sake of leaving the organic membrane in a 

 state adapted to separate examination, Hydrofluoric acid must be 

 employed as the menstruum. It is sometimes necessary to get rid 

 of the Organic matter, for the sake of obtaining the Mineral par- 

 ticles in a separate state, as in the case of the spicules of Sponges, 

 Grorgonige, &c. : this may be done either by incineration, or (which 

 is generally preferable) by boiling or by macerating for a long time 

 in a solution of caustic potash. In sepai'ating from Guano, again, 

 the Siliceous skeletons of Diatomacese, &c, which it may contain, 

 Hydrochloric and Nitric acids are largely used to dissolve away every 

 part of the mass on which they will act ; the microscopic organisms 

 for which search is made being contained in the few grains of 

 sediment which are left when a pound of pure guano has been 

 thus treated. — On the other hand, it is often desirable to harden 

 Animal Tissues, in order that they may be more readily examined : 

 this is best effected in some instances by maceration in strong 

 Alcohol,* and in others by maceration in a solution of Chi'omic 

 Acid, so dilute as to be of a pale straw colour, which is particu- 

 larly efficacious in bringing into view the finer ramifications of 

 Nerves. 



143. In applying Chemical Re-agents to Microscopic objects for 

 the purpose of testing, it is necessary to use great care not to add 

 too much at once ; and the Test-Bottle itself may be made to afford 

 the means of regulating the quantity, in either of the following 

 modes : — The stopper of the test-bottle may be drawn to a capil- 

 lary orifice, from which the fluid is caused to flow, drop by drop, 



* The Author has found this menstruum especially useful in his 

 researches into the structure of Comatula, the tissues of which, when 

 fresh, are so extremely soft that their parts are almost undistinguishable. 



