PRESERVATION OF OBJECTS. 785 



ferous trees, as the pine, this section shows also the beautiful puncta- 

 tions on the walls of the fibres. The tangential-vertical section ia a 

 slice across the ends of the medullary rays, and exhibits the form and 

 arrangement of the cellular tissue in them. The cells of the rays are 

 seen projecting between the fibres of the wood. These vertical 

 sections show the form, size, and connections of the woody tubes and 

 the spiral, reticulated, and dotted vessels. In endogenous trees hori- 

 zontal and vertical sections are also required. Peat wood requires to 

 be digested in a strong solution of carbonate of soda, and fossil woods 

 which have .been converted into carbonate of lime should be digested 

 in dilute hydrochloric acid (1 of acid to 20 of water). 



Schleiden gives the following method of preserving minute struc- 

 tures for the microscope. Upon a glass slide of the common form two 

 narrow slips of paper are gummed, of a thickness proportioned to the 

 object, and at a distance which is regulated by its size. Between 

 these the object is laid in a drop of solution of chloride of calcium 

 (60 grains to half-an-ounce of water). A thin slip of glass, sufficient' 

 to cover the object and paper slips, is put on ; the slips 

 are gummed, and the thin glass applied to its place, 

 where it is retained by the gum drying. The whole 

 may be secured by pasting a long slip of paper over 

 all, with a hole for the object. The method has the 

 advantage of preventing all running in, which is so apt 

 to happen when asphalte varnish is employed. Ohloride 

 of calcium, being deliquescent, never dries up, and, if 

 evaporation takes place, water is easily introduced at 

 the open sides of the thin glass. The points to be 

 attended to are — 1, that the paper between the glasses 

 be thick enough to prevent much pressure on the 

 object, and not so thick as to allow it to float about or 

 fall out at the side ; 2, that the drop of solution be not Kg. 953. 

 too large, but covering the object, and yet not reach- 

 ing the paper. Glycerine may be used in place of chloride of calcium 

 in cases where the objects are very delicate, or contain chlorophyll or 

 albumen. 



SmaU specimens for the microscope, such as Diatoms and Desmidiese, 

 and many small Seaweeds, as well as vegetable tissues, are put up on 

 slides (fig. 953), in the centre of which there is a circular cavity formed 

 by a layer of asphalte,* and covered by a circular piece of tldn glass. 



Fig. 953. Glass slide for microscopic preparations, 3 inches long and 1 inch broad. In 

 the centre is a ring of asphalte, forming a cell to contain fluid ; the object marked by a + 

 in the centre is covered by a circular piece of thin glass fitted to the asphalte rim. The 

 name of the object is often written on the glass, but perhaps it is preferable to "write the 

 name on coloured paper, and attach it to the glass by isinglass or fine bookbinder's glue. 



* Prepared asphalte is better than gold size or black japan varnish, as it dries more 

 rapidly, and is less liable to run, It can be procured from opticians. 



3 E 



o 



SPIRAL CELLS 

 ORCHID. 



