44 EXPERIMENTS ON THE 



The following letter from Dr. Dalton to Mr. Clegg points out the methods 

 of detecting some impurities : 



"Manchester, Jan. 2nd, 1815. 



" RESPECTED FRIEND, I should have answered thine inquiries immediately if I had 

 any observations to make of any importance, but I have little of anything to offer more 

 than has been already published in my book ; thou art also acquainted with Dr. Henry's 

 paper in ' Nicholson's Journal,' a few years ago. 



" It is remarkable that no defiant gas* ( I believe) is obtained from coals, though they 

 contain all its elements. If it is to be obtained at all, it must be by a minimum of heat, 

 or by using some additional article. Oxymuriatic gas is the best thing for detecting it : 

 when the gases are put together an immediate diminution and the formation of oil ensues. 

 The diminution with carburetted hydrogen is slow and invisible, unless in a strong light. 

 I should put a weak solution of acetate of lead into a given volume of coal-gas, and agi- 

 tate it, to find the sulphuretted hydrogen ; a black sulphuret of lead would immediately fall 

 down, and from the weight of it, that of the sulphuretted hydrogen might be inferred. I 

 believe arseniuretted hydrogen may sometimes occur in coal-gas ; if the coals contain any 

 arsenic, it must, I apprehend, as this metal and hydrogen unite and are volatile. This 

 gas loses its arsenic by electricity and long standing, it falls as a black powder, or particle, 

 and loses its volume of hydrogen ; it is absorbable by water in the same degree as coal-gas. 



"Thine, etc. . 



Samuel Clegg." " JOHN DALTON." 



The quantity and description of coke yielded by a specimen of coal cannot 

 be ascertained by the apparatus just described, and, if desired, it must be 

 done in one of the working retorts, weighing the coal when it is charged, and 

 re-weighing the coke when drawn ; the quantity ought to be about from 38 Ibs. 

 to 44 Ibs. per 56 Ibs. of coal. It should be granular and compact, the particles 

 shining with somewhat of a silvery lustre, and when exposed to a white heat 

 it should leave no white or brown ashes. The following is an average result 

 of five experiments made after the plan just described : 



August, 1821. Heat of experimental retort, 27 Wedgewood. 3^ Ibs. of 

 Berwick and Craister's Wall's-end coal yielded in 10 minutes 16^ cubic feet 

 in 20 minutes 20 cubic feet. Sp. gr. of first portion 0'47l ; of second por- 

 tion - 432 : burnt from an Argand lamp consuming 6*2 cubic feet of gas per 

 hour. The first portion gave a light equal to that of twelve candles called 



* Dr. Dalton at that period was not aware of olefiant gas being evolved during the production 

 of coal-gas. Dr. Henry detected its presence by the test of the action of chlorine. 



