954 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Hydrofluoric Acid, HF, is prepared by heating a fluoride with concentrated sul- 

 phuric acid, in a platinum or lead dish, 



CaF 2 + H 2 S0 4 = CaS0 4 + 2HF. 



Properties. Hydrofluoric acid is a colorless gas, so powerfully corrosive that breathing 

 its fumes results fatally. Its aqueous solutions are stable, but can be kept only in vessels 

 of platinum, gold, lead, or india-rubber. It etches glass, uniting to form volatile silicon 

 fluoride, 



Detection. If silicon be absent from the substance to be tested, the above reaction may be 

 used, and if the glass be etched, after treating the substance with sulphuric acid, fluorine 

 is present. In the organism silicon is found, and the method of detection is different. 

 The principle of the method depends on the fact that SiF 4 in contact with water forms 

 silicic acid (H 4 Si0 4 ), and hydrofluor-silicic acid (H 2 SiF 6 ). If the ash of the organ be 

 mixed with powdered silica (Si0 2 ), transferred to a flask, mixed with concentrated sul- 

 phuric acid, then heated, and if a current of dry air remove the SiF 4 from the flask 

 through a tube into water, the slightest trace of fluorine is proven by the appearance 

 of a whitish cloud of silicic acid at that part of the tube where SiF 4 first comes in con- 

 tact with moisture. This may be noted when 0.0001 gram of fluorine is present. 1 



CIRCULATION IN THE BODY. Tappeiner and Brandl 2 have shown, on 

 feeding sodium fluoride (NaF) to a dog in doses varying between 0.1 and 

 1 gram daily, that the fluorine fed was not all recoverable in the urine and 

 feces, but was partially stored in the body. On subsequently killing the dog, 

 fluorine was found in all the organs investigated, and was especially found in 

 the dry skeletal ash to the extent of 5.19 per cent, reckoned as sodium fluoride. 

 From the microscopic appearance of the crystals seen deposited in the bone, the 

 presence of calcium fluoride was concluded. In this form it normally occurs 

 in bones and teeth. 



NITROGEN, N = 1 4. 



Free nitrogen constitutes 79 per cent, of the volume of atmospheric air. It 

 is found dissolved in the fluids and tissues of the body to about the same extent 

 as distilled water would dissolve it. It is swallowed with the food, may par- 

 tially diffuse through the mucous membrane of the intestinal tract, but forms 

 a considerable constituent of any final intestinal gas. It is found in the atmos- 

 phere combined as ammonium nitrate and nitrite, which are useful in furnish- 

 ing the roots of the plant with material from which to build up proteid. 

 Bacteria upon the roots of certain vegetables combine and assimilate the free 

 nitrogen of the air (Hellriegel and Willforth). Cultures of algae do the same. 3 



Preparation. (1) By abstraction of oxygen from air through burning phosphorus in 

 a bell jar over water, pentoxide of phosphorus being formed, which dissolves in the water 

 and almost pure nitrogen remains. 



(2) By heating nitrite of ammonium, 



NH 4 NO a = 2N + 2H 2 0. 



Properties. Nitrogen is especially distinguished by the absence of chemical 

 affinity for other elements. It does not support combustion, and in it both a 



1 Tammann, loc. cit. 2 Zdtechrift fur Biologic, 1892, Bd. 28, p. 518. 



8 P. Kossowitch : Botanische Zeitung, 1894, Jahrg. 50, p. 97. 



