Improved Form of Coal-Calorimeter . 455 



cent, ash, the time of a combustion will be about six minutes, 

 and the rise of temperature about 2J° C, but as the com- 

 bustion takes place in full view, it can be readily retarded or 

 accelerated by adjusting the oxygen. 



In the author's opinion, the present form of coal-calori- 

 meter has certain special advantages, and these will best 

 appear by comparing it with the well-known Mahler-Ber- 

 thellot "bomb" calorimeter. Certain chemists have stated 

 that the latter form of instrument, in which the coal is burnt, 

 practically instantaneously, in an atmosphere of oxygen 

 under very high pressure (up to 180 lbs. per sq. in.) is alone 

 reliable, as in their opinion * the combustion of coal in 

 a current of oxygen under atmospheric pressure is never 

 complete. Were it possible or necessary to determine the 

 calorific value of coal to an accuracy of 0*1 per cent., then 

 this objection would be perfectly valid, as such combustion is 

 never perfectly complete, but such accuracy would in any 

 case be merely imaginary, owing to the varying quality of 

 the coal in various parts of even one truck-load, and the 

 author has found that by observing certain precautions, the 

 -combustion in the present instrument can be rendered suffici- 

 ently complete for the degree of accuracy required. The 

 most important of the special precautions which must be 

 observed is to provide for a free access of the oxygen to the 

 entire sample of fuel. When the coal is burnt as a powder 

 lying in a platinum crucible, this condition is not fulfilled, as 

 the carbon dioxide formed by the combustion tends to remain 

 in the crucible and to dilute the inflowing oxygen. To 

 avoid this, the author replaces the platinum crucible by a 

 flat tray of porcelain, and uses the sample of coal in the 

 shape of a small cylindrical briquette standing freely on the 

 tray, the briquette being readily formed from the powdered 

 sample by pressure in a mould. The author prefers a porce- 

 lain to a platinum tray on account of the feebler heat- 

 conductivity of the former, which therefore does not cool 

 down the layers of fuel in contact with it so rapidly towards 

 the end of the combustion. The author finds that these 

 modifications reduce the time occupied by a combustion to 

 about one half, and at the same time cause a very nearly 

 complete combustion to take place. The only residue is 

 generally a very thin film of tarry matter deposited upon the 

 porcelain tray close to the specimen. To determine the 

 weight of this unburnt residue the following experiments were 

 made. A series of samples of coal were burnt in the calori- 

 meter under working conditions, except that larger quantities 



* Hempel, ' Gas Analysis,' p. 356. 



