REAGENTS AND PROCESSES 547 



Hoyer's picro-carmine solution is made by dissolving carmine in a concentrated 

 solution of neutral ammonium picrate. A solution of carmine and picric acid is 

 known as Picro-carmine Solution. Carmine solutions give with cellulose, the 

 nucleus and proteins a red color. 



Hydrochloric Acid. This reagent has such manifold application in histology 

 that its uses are best learned in the specific cases of its application. See in the next 

 chapter under Amylose, Berberin, Caffeine, Calcium Oxalate, Calcium Sulphate, 

 Ethereal Oils, Elasocapsin, Magnesium Sulphate, Middle Lamella, Myrosine, Pectic 

 Substances, Phloroglucin, Theobromine, Vanillin. See also in this chapter under 

 Maceration. 



Hydrogen Peroxide. One part of hydrogen peroxide mixed with 20 parts of 

 60 per cent, alcohol will, in a few minutes, remove from sections the dark discolora~ 

 tion due to osmic acid which has been used as a fixative. 



Infiltration. The stony tissues of seeds, etc., which are too hard and brittle 

 to be sectioned with a knife, and must, therefore, be ground to the requisite 

 thinness on a stone or by means of emery powder, may be protected against 

 breaking during this process if fairly thin sections are first cut with a fine saw 

 and then placed in a rather thin solution of Canada balsam or copal in chloroform, 

 which is then allowed to evaporate to the thickness of syrup; the sections are 

 allowed to dry and are then cemented by means of a thick solution of gum arabic 

 to a glass plate preparatory to grinding. Only a thin layer of gum arabic should 

 be used, and this should be quite dry before the grinding is begun. The sections 

 may be ground thin on a clean, dry Arkansas or Wichita stone. Before the sec- 

 tion has been brought to the desired thinness, the surface should be polished by 

 rubbing it on a piece of soft leather which has been dressed with tripoli. The stone 

 on which the sections are ground may be cleaned of the balsam from time to time 

 by means of a cloth dipped in xylol or turpentine. When one side has been polished, 

 the section may be freed from the glass plate by soaking in water, and then the 

 polished side should be cemented to the glass plate and the reverse side ground and 

 polished as before. The sections should be examined from time to time with the 

 microscope, so that the process of grinding may be stopped as soon as the desired 

 transparency has been obtained. They may then be washed from the glass plate 

 with water, and after drying should be mounted in Canada balsam. 



Iodine. The fumes from heated crystals of iodine serve well in many cases as 

 a fixative. Small objects in drop cultures may be readily fixed by pouring over 

 them the fumes arising from iodine heated in a test-tube. Algae may be fixed by 

 placing a few crystals of iodine in the bottom of a test-tube, cautiously inclining 

 the tube slightly with the mouth downward, then placing the algae in the test-tube 

 near the mouth directly from the water in which they were growing, and there- 

 after heating the crystals so that the fumes from them pour down over the algae. 

 The iodine may afterward be expelled by warming the fixed material to 30 or 

 40 C., and the material will then need no further washing out. 



Iodine has a wide application in plant histology and microchemistry. See 

 under Aconitine, Atropine, Carotin, Cellulose, Colchicine, Gums, Gram's Method, 

 Lipochromes, Lignin, Mucus-globules, Nicotine, Proteids, Suberin. 



Iodine and Alcohol. A good fixative for very small organisms is a solution 

 of 3 parts of iodine in 100 parts of 70 per cent, alcohol. This, at the same time, 

 permits the staining effect of iodine on the cell- wall and cell-contents. 



Iodine and Aluminum Chloride. Aluminum is dissolved in hydrochloric acid 

 to saturation, and then allowed to evaporate to the consistence of syrup. Cellulose 

 is colored a dark blue to violet color when successively acted on by this reagent and 



