202 THE MICROSCOPE AND ITS REVELATIONS. 



other deposits, that sections of them should not be cut until they have 

 been softened by being partially or wholly freed from these. Accordingly, 

 pieces of stems or roots should be soaked for some days in water, with 

 the aid of a gentle heat if they are very dense, and should then be steeped 

 for some days in methylated spirit, after which they should again be 

 transferred to water. The same treatment may be applied to hard-coated 

 seeds, the ' stones' of fruit, ' vegetable ivory,' and other like substances. 

 Some Vegetable substances, on the other hand, are too soft to be cm, 

 sufficiently thin without previous hardening, either by allowing them to 

 lose some of their moisture by evaporation, or by drawing it out by steep- 

 ing them in spirit. Either treatment answers very well with such sub- 

 stances as that which forms the tuber of the Potato; sections of which 

 display the starch-grains in situ. Where, on the other hand, it is desired 

 to preserve color, spirit must not be used; and recourse may be had to 

 Gum-imbedding ( 191), which is particularly serviceable where the 

 substance is penetrated by air-cavities, as is the case with the Stem of 

 the Rush, the thick leaves of the Water-lily, etc. But where the stain- 

 ing process is to be employed ( 200), the substance should be previously 

 bleached by the action of chlorine (preferably by Labarraque's chlorinated 

 soda), and then treated with Alcohol for a few hours. 



199. Hardening of Animal /Substances. Save in the case already 

 treated of ( 192), in which the tissues are consolidated by Calcareous 

 deposit, the preparatory treatment of Animal substances consists in 

 hardening them. The very soft tissues of which most of the lower 

 Animals are composed, contain so large a proportion of Water, that the 

 withdrawal of this by immersion in strong spirit causes them to shrink so 

 much as completely to obscure their structure. Nothing has yet been 

 found so serviceable in preserving them as Osmic acid ; the poisonous 

 action of which at once kills living Infusoria, etc., Echinoderm or 

 Annelid larvae, and the like; and hardens their delicate organisms, so as 

 to allow them to be afterwards stained and preserved with very little 

 change; and thus many points of their structure can be better made out 

 in their ' mounted > than in their living state. The special procedures 

 which have been successfully worked out by M. Certes for Infusoria, and 

 by Mr. Percy Sladen for Echinoderm larvce, will be described under those 

 heads. The hardening of the general body-substance of the larger 

 Invertebrata is for the most part sufficiently effected by the action of the 

 Alcoholic spirit in which they are usually preserved; and this may be 

 carried farther, if required, by steeping them for a time in absolute 

 Alcohol. For hardening particular tissues, however, such as Nerves, 

 recourse must be had to some of those hardening agents, used in the 

 preparation of the Tissues of the higher Animals, which will be now 

 specified : 



a. Alcohol. For hardening purposes, Rectified spirit should be used in pref- 

 erence to methylated; and its action is (as a rule) most beneficial after some of 

 the other hardening agents have been employed. The substance to be hardened 

 should be first placed for a day or two in a mixture of equal parts of rectified 

 spirit and water, then transferred for about 48 hours to rectified spirit, and 

 from this to absolute alcohol. One injurious effect of this treatment is, that by 

 the coagulation of their albuminous components many textures are rendered 

 opaque: but, as Dr. Beale pointed out, this may be corrected by the addition 

 of a little caustic Soda, which must be made, however, with great caution. 

 When the Alcoholic treatment is used merely for so dehydrating sections pre- 

 viously immersed in watery solutions, that they may be mounted in Canada 

 balsam or Dammar, they may be transferred at once from rectified spirit to oil 

 of turpentine, without treating them with absolute alcohol. 



