168 Dr. Thomson ofi Coal Gas. 



smallest quantity 2-^1 per cent: the quantity in the three gases 

 from cannel coal was as follows : — 



Skaterigg . . 12*29 



Lesmahagow. 11*46 



Monkland . . 17*32 

 The common method of determining the light emitted by 

 gas during its combustion, is to set fire to a jet of a given 

 height and issuing from an orifice of a given diameter, and to 

 compare it with the light given out by a wax candle of six in 

 the pound, usually denominated short sixes ; an opake body 

 is placed on a sheet of paper, horizontally between the two 

 flames, and it is so placed that the two shadows formed by it 

 are of equal intensity. The distance between this opake body 

 and the flames is measured, and the light emitted by each is 

 as the square of that distance; thus, if the distance between 

 the gas flame and the opake body be two feet, while its di- 

 stance from the flame of the candle is only one foot, then the 

 light given out by the gas is four times as great as that of the 

 candle. 



The light given by the combustion of a jet of Glasgow gas, 

 issuing from an orifice of ^\jth of an inch in diameter and four 

 inches in height, was as follows: — 



1. On the north side of the river =2*68 candles. 



2. On the south side of the river 1*77 



This method of measuring the quantity of light appears at 

 first sight very simple, but I found on trial that it was attended 

 with so many sources of error that I was afraid to depend 

 upon it. Fortunately there is another method of much easier 

 execution, which I found much more satisfactory. 



The quantity of light given out during the combustion of 

 coal-gas is very nearly proportional to its specific gravity ; the 

 heavier a gas is the slower does it issue from an orifice of a 

 given diameter when propelled by a given force. I measured 

 the time which a cubic foot of each gas took to issue from an 

 orifice of ^\jth of an inch, when propelled by a force such as 

 to form a jet of flame, when lighted, of four inches in length, 

 and I considered the goodness of the gas as proportional to 

 this time: the result was as follows: — 



1. Glasgow gas, north . 70 18 



2. ... south .60 9 



3. Skaterigg 84; 10 



4. Lesmahagow 101 10 



5. Monkland 101 



Certainly, in a commercial point of view, the value of the 

 gas (the price per cubic foot being the same in all) is exactly 



