FERROCYANIDES 355 



sheltered from the air, or in the presence of limited quantities of pure air, it does not 

 give alien fermentations, because the yeast of beer only was sown ; and whatever is 

 advanced on the subject of possible transformation of yeast into Mycoderma aceti, com- 

 mon mildews, &c., or vice versa, is erroneous. Lastly, when the beer is finished, it can 

 be treated in the ordinary manner, without any fear of the contact with the air pro- 

 ducing any serious inconveniences, because the beer finished or on the point of being 

 finished, does not offer favourable conditions for receiving the germs of disease ; at 

 least, those which do not want the oxygen of the air to live and multiply. As to the 

 others, which are the Mycoderma aceti and the Mycoderma vini, simple precautions 

 enable them to be easily avoided. 



' Beer made under the conditions I have shown, and placed according to custom 

 in casks recently tarred, or put in bottles, may be preserved for an indefinite time, 

 even in a stove of from 20 to 25 C. Far from suffering any change by time, it 

 appears rather to become softened by the effect of old age, in the same manner as 

 wines which can be preserved without deteriorating. One can now understand the 

 possibility of suppressing the use of ice during and after the fermentation, since the 

 new process is applicable in all temperatures. The temperature of the cellars where 

 the beer is preserved should be not lower than 10 to 12 C. ; a temperature that can 

 be obtained, even in summer, in temperate climates, by having the cellars a moderate 

 depth.' 



FERMENTATION PAX ART. See BREAD. 



FERRIC ACID. (FeO 3 .) This compound having been prescribed as a source 

 of supplying oxygen to persons confined in diving-bells and in mines, by M. Payerne, 

 claims notice in a practical work. M. Fremy is the discoverer of this acid, which 

 he obtains in the state of ferrate of potash, by projecting 10 parts of dry nitre in 

 powder upon 5 parts of iron filings, ignited in a crucible ; when a reddish mass, 

 containing much ferrate of potash, is formed. The preparation succeeds best when 

 a large crucible, capable of holding about a pint of water, is heated so strongly that 

 the bottom and a couple of inches above it, appear faintly, but distinctly red, in 

 which state the heat is still adequate to effect due deflagration without decomposition. 

 An intimate mixture of about 200 grains of dried nitre with about one-half its weight 

 of the finest iron filings, is to be thrown at once upon the side of the crucible. The 

 mixture will soon swell and deflagrate. The crucible being taken from the fire, and 

 the ignited mass being cooled, is to be taken out with an iron spoon, pounded, and 

 immediately put into a bottle, and excluded from the air, from which it would speedily 

 attract moisture, and be decomposed. It is resolved by the action of water, especially 

 with heat, into oxygen gas, peroxide, and nitrate of iron. This acid has not been 

 obtained in a free state ; it appears indeed to be scarcely capable of existing alone, 

 decomposing, as soon as liberated, into oxygen and ferric oxide. Graham. 



Mr. J. D. Smith prepares the ferrate of potash by exposing to a full red heat a mix- 

 ture of finely -powdered peroxide of iron with four times its weight of dry nitre. It 

 has an amethyst hue, but so deep as to appear black, except at the edges. Oxygen is 

 rapidly evolved by the action of sulphuric or nitric acid upon its somtion. Ho 

 considers the atom of iron to exist in this compound associated with 3 atoms of 

 oxygen, or double the proportion of that in the red oxide. Henca 52 grains of pure 

 ferric acid should give off 12 grains of oxygen, equal to about 35 cubic inches; but 

 how much of the ferrate of potash may be requisite to produce a like quantity of oxygen 

 cannot be stated, from the uncertainty of the operation by which it is produced. 



FERRIC All CITE. A term sometimes applied to those limestones which con- 

 tain a large percentage of oxide of iron. The iron ore 'Ked ore,' of the Churnet 

 Valley, North Staffordshire may be regarded as a striking example. 



FERRIC SAIiTS. Salts of ferric oxide, or peroxide of iron. Thus the sulphate 

 of sesquioxide of iron may be called ferric sulphate. 



FERROCYANTDES. The compounds of the radical ferrocyanogen. The latter 

 radical is bibasic ; when, therefore, it combines with hydrogen to form ferrocyanic 

 acid, it takes up two atoms. These two atoms of hydrogen can be replaced by metals 

 as in ferrocyanide of potassium or prussiate of potash, as it is commonly called. See 

 PRTJSSIATE OF POTASH. Ferrocyanogen consists of C 6 N 8 Fe, which may also be written 

 Cy 3 Fe, or, for brevity's sake, Cfy. 



The modes of preparing the ferrocyanides differ, according as the resulting sub- 

 stance is soluble or insoluble in water. The soluble salts, such as those with alkalis, 

 are prepared either by neutralising hydroferrocyanic acid with the proper metallic 

 oxide, or by boiling prussian blue with the oxide, the metal of which it is intended to 

 combine with the ferrocyangen. Other methods may also be adopted in special cases. 

 The processes for preparing the ferrocyanides of the alkali-metals on the large scale 

 will be described in the article PEUSSIATE OF POTASH. 



When the ferrocyanide is insoluble in water, it may be prepared by precipitating 



A A 2 



