674 CALOTYPE 



oxide of manganese, and sulphuric acid, or of binoxido of manganese and muriatic 

 acid in order to produce chlorine gas. 



The mode of operating with this apparatus is as follows : A quantity of mercury 

 or quicksilver is placed in a glass vessel, and the temperature of the same is raised to 

 between 350 and 660 F., by means of an open fire beneath. The chlorine gas, as it 

 is generated, passes from the alembic through the bent tube into the glass vessel, and 

 there combining with the vapour of the mercury, forms either corrosive sublimate or 

 calomel, according to the quantity of chlorine gas employed. 



The product is found at the bottom of the air-tight chamber, and may be removed 

 from the same through a door, when the operation is finished. 



Prof. Wohler proposes to prepare calomel in the humid way by decomposing a solu- 

 tion of corrosive sublimate by sulphurous acid. The commercial salt is dissolved in 

 water at 122 to saturation. Sulphurous acid gas, evolved by heating coarse charcoal 

 powder with concentrated .sulphuric acid, is passed into the hot solution : the separa- 

 tion of the calomel commences immediately. When the solution is saturated with 

 the gas, it is digested for some time, then left to get cold, and filtered from the calomel, 

 which is afterwards washed. The filtrate usually contains some unchangeable 

 corrosive sublimate, which may be converted into calomel, either by heating to 

 boiling, or by a fresh introduction of sulphurous acid and heating. Calomel obtained 

 in this manner is a crystalline powder of dazzling whiteness, glittering in the sun- 

 light 



The presence of corrosive sublimate in calomel is easily detected by digesting 

 alcohol upon it, and testing the decanted alcohol with a drop of caustic potash, 

 when the characteristic brick-coloured precipitate will fall, if any of that salt be 

 present. To detect subnitrato of mercury in calomel, digest dilute nitric acid on it, 

 and test the acid with potash, when a precipitate will fall in case of that contamina- 

 tion. 



117'75 parts of calomel contain 100 of quicksilver. 



Great confusion has unfortunately arisen between the formulae of the two chlorides 

 of mercury calomel and corrosive sublimate. 



If 100 be taken as the atomic weight of mercury, the formula of calomel is Hg*Cl 

 (as given above), and that of corrosive sublimate is HgCl. But if, as is often the 

 case, 200 be assigned to mercury as its atomic weight, the formula of calomel 

 becomes HgCl (or rather, for theoretical reasons, Hg 2 Cl 2 ) and that of corrosive 

 sublimate HgCl*. As fatal mistakes might occur by substituting one of these com- 

 pounds for the other, the formulae should never be used apart from the specific names. 



CAIiORIC. A term once extensively used in physical science, and perhaps some- 

 what too hastily abandoned. When employed there was a very general impression 

 that heat was the effect of some undeveloped cause, and to this cause the term caloric 

 was applied. The modern hypothesis regarding heat as a mode of motion, the term 

 has been dispensed with. See HEAT. 



CAXiORIFERE OF WATER. In former editions, hot-water apparatus was 

 described under this head : now see HOT-WATER APPARATUS. 



C AXjOROnviETER. An instrument intended to measure the quantity of heat pro- 

 duced by burning bodies, devised by Lavoisier and Laplace. The principle upon 

 which these instruments depended may be easily understood. To convert a certain 

 quantity of ice into water, a given amount of heat is necessary. Now suppose we 

 construct a funnel-shaped vessel, and having placed in the middle of it a vessel con- 

 taining boiling water, or hot mercury, or oil, or a red-hot piece of metal, it is packed 

 round with ice, care being taken that no heat shall escape, except it is employed to 

 thaw the ice. The water formed by the ice is collected in a vessel placed below the 

 funnel, and the quantity collected during the cooling of the heated body will represent 

 the heat given out and employed in liquefying the ice. For nice experiments certain 

 exact determinations are required, for which, and the general rules to be observed, 

 some good treatise on Physics should bo consulted. See Watts's ' Dictionary of Chemis- 

 try : ' article HEAT. 



CAX.OTYPE (signifying beautiful picture). A name given by Mr. Henry Fox 

 Talbot to a photographic process invented by him in 1840, and patented in 1841. 



Mr. Talbot's description of his process is as follows : 



Take a sheet of the best writing-paper, having a smooth surface, and a close and 

 even texture. The water-mark, if any, should be cut off, lest it injure the appearance 

 of the picture. Dissolve 100 grains of crystallised nitrate of silver in six ounces of 

 distilled water. Wash the paper with this solution with a soft brush on one side, and 

 put a mark on that side, whereby to know it again. Dry the paper cautiously at a 

 distance from the fire, or else let it dry spontaneously in a dark room. When dry, or 

 nearly so, dip it into a solution of iodide of potassium, containing 500 grains of that 

 salt dissolved in one pint of water, and let it stay two or three minutes in the solution. 



