ing to from fifty to seventy-five per cent of the original mass, to fall throiigli into a bin, from which it is collected to be used again. The lime requires to be fairly pure, over five per cent of impurities interfering seriously with the production of carbide. Magnesia is very undesirable, and it is stated that if over three per cent is present a good quality of carbide can not be made. This matter of impurities and the care of the carbon pencils, which when properly looked after wear away only about 0"09 of an inch per hour, but which may make a great deal of trouble if carelessly tended, are the points requiring special attention. Un- slaked lime is said to give the best results. The alternating cur- rent is used at Spray, but a direct current can be employed. Be- sides coke, soft coal, anthracite, charcoal, pitch, tar, resin, and asphalt have been tried in combination with lime. Indeed, the first mixture used by Mr. Wilson was lime and tar, which had been boiled together in a caldron and then thoroughly dried. With the exception of charcoal, however, none of these substances were found of any value as compared with coke, although they all produced some carbide. As regards the amount and quality of the light obtained from acetylene properly burned, there seems no question as to its great superiority over either ordinary coal or water gas. It stands about even with water gas in poisonous qualities, but is more liable to explosion. Greater care would be required in handling it, especially if the proposition, which was at first well received, to use it in a liquefied state from cylinders under great pressure, should prove practicable.
Its success as an illuminant, however, depends almost entirely on the cost of manufacture, and regarding this point it is some- what difficult at present to get reliable data, chiefly, perhaps, be- cause of the experimental stage in which much of the apparatus still is.
The Progressive Age, a New York publication devoted to the interests of electricity, gas, and water, recently formed a commis- sion which it sent down to Spray for the purpose of determining the actual cost of manufacturing calcium carbide. The commis- sioners were Prof. Houston and Drs. Kennelly and Kinnicutt, and their conclusions, after careful examination of the works and a testing of two full runs, were published in the Progressive Age for April 15th, and are as follows :
" Our estimate, therefore, of the cost of producing calcium car- bide at Spray, by working the furnaces three hundred and sixty- five days a year and twenty-four hours a day, yielding on the average one ton of two thousand pounds gross carbide a day, is $32.76 per ton. Of this amount $14.39 is for material. The freight charges on lime and coke are heavy at Spray, and add materially to the cost."