212 PREPARATION OF OBJECTS. 



113. In applying Chemical Eeagents to Microscopic objects 

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

 add too much at once ; and it is better that the test-bottle itself 

 should afford the means of regulating the quantity, than that an 

 additional rod or tube should be required. Two modes have 

 been devised for this purpose. One consists in drawing the neck 

 of the test-bottle to a capillary orifice, and covering it with a cap 

 which fits around it ; and the fluid is caused to flow from this, 

 drop by drop, by the warmth of the hand applied to the bottle, 

 which causes an expansion of the air it may contain.' When 

 these bottles are emptied, they must be refllled by expelling the 

 air by heat, and placing the capillary oriflce under the surface of 

 the fluid to be introduced, which will then be forced in as the 

 bottle cools ; this process may need to be repeated two or three 

 times (care being taken that the heat applied be not so great as 

 to crack the bottle) ; but it is better not to flll the bottle more 

 than half-full, in order that air enough may be left for the warmth 

 of the hand to act upon. The other arrangement for applying 

 minute quantities of test-liquids, consists in the elongation of the 

 stopper, which is drawn to a fusiform point, so as to serve as the 

 test-rod for its own bottle.^ This enables either a mere trace, or 

 several ordinary drops, of the reagent to be applied at once ; for 

 the elongated stopper will take up a considerable quantity, a 

 larger or smaller proportion of which (as desired), may be left 

 behind, by bringing the lower part of the stopper into contact 

 with the inside of the neck of the bottle, as it is being withdrawn. 

 Whichever plan is made use of, great care should be taken to 

 avoid carrying away from the slide to which the test-liquid is 

 applied, any loose particles which may be upon it, and which 

 may be thus transferred to some other object, to the great per- 

 plexity of the Microscopist. It is better, indeed, not to deposit 

 the drop of test-liquid on the slide in immediate contact with the 

 substance to which it is to be applied ; but to bring the two into 

 contact after the test-bottle has been withdrawn. 



114. The following are the Test-Liquids most frequently 

 needed. 



1. Solution o^ Iodine in water (1 gr. of iodine, 3 grs. of iodide 

 of potassium, 1 oz. of distilled water) turns stai'ch blue, and cel- 

 lulose brown ; it also gives an intense brown to albuminous sub- 

 stances. 



2. Dilute Sulphuric Acid (one of acid to two or three parts of 

 watei') gives to cellulose that has been previously dyed with 

 iodine, a blue or purple hue; also, when mixed Avith a solution 

 of sugar, it gives a rose-red hue, more or less deep, with nitro- 

 genous substances and with bile (Pettenkofer's test). 



' A set of 12 test-bottles on this plan, packed in a box, is supplied by Mr. Highley, of 

 Fleet Street. 



2 Bottles of this pattern, which was devised by Dr. Griffith, are sold by Mr. Fergusoa 

 of Giltspur Street. 



