48 



repeat the process until we get a satisfactory result. The chloride of lime 

 should be as freshly prepared as possible, and therefore it will be best to 

 purchase it as we require it. It should be a perfectly dry powder — if at 

 all moist it will be useless for our purpose. 



If the sections have been preserved in dilute spirit, they must be floated 

 on distilled water, and, when they have all sunk to the bottom, as much of 

 the water as possible must be drained off, and a considerable quantity of the 

 bleaching fluid poured over them. Examine the sections from time to time, 

 holding the vessel containing them against a white background ; a white 

 china tile, such as can be bought at any artists' colourman for a few pence, 

 answers admirably. Do not allow them to remain in the bleaching fluid 

 longer than is necessary, but, directly they are quite bleached, pour off the 

 fluid and fill the vessel up with clean water. Repeat the washing at least 

 five or six times, allowing the sections to remain in the water for at least 

 12 hours, and using distilled water for the last washing; then preserve 

 them in a mixture of equal parts of methylated spirit and water until they 

 are required for staining. 



In order to stain the sections with logwood, the first thing, of course, is 

 to procure a suitable staining fluid, and for this purpose a preparation in- 

 vented by Dr. Mitchell, of Philadelphia, and described in " The Science 

 Monthly " for March last, gives by far the finest results of any stain I have 

 yet seen. The principles involved in its preparation are as follows : Log- 

 wood contains, besides the colouring matter, considerable quantities of 

 tannin, and vegetable infusions containing tannin are quickly influenced 

 by the action of light and air. This is the cause of the muddy sediment 

 deposited by most logwood stains, the colouring matter being also seriously 

 affected by the decomposition. Now, tannin is readily soluble in cold 

 water, whilst the colouring matter is but very slightly soluble in that 

 fluid ; hence, by washing the logwood, finely divided, with cold water, the 

 injurious tannin will be removed without the staining properties of the 

 logwood being materially affected. 



Dr. Mitchell's directions, slightly modified, are as follows : First grind 

 up some logwood chips in a coffee-mill. Then place the ground chips in a 

 linen bag in a percolator, and pour cold distilled water over them until the 

 liquid coming through is very slightly coloured and has no astringent taste. 

 Then squeeze out as much of the water as possible, and spread the log- 

 wood on a plate to dry. 



Take of the dried prepared chips, 1 oz. 



Ground potash alum, 4^ drs. 



Distilled water, 6 fl. oz. 



Glycerine, 2 fl. oz. 



Dissolve the alum in the water, then add the glycerine and mix 

 thoroughly. Macerate the logwood in this mixture for 48 hours, stirring at 

 intervals, filter the resulting stain, and preserve it in a stoppered bottle. 



Not only does the stain, thus prepared, give a very beautiful colour to 

 the sections, cool to the eyes, and wonderfully sharp in outline ; but it is 

 also so selective that the various kinds of tissue are really differentiated, so 



