202 Heat of Combustion of Organic Compounds. 



the heat of combustion and the vapour density d are known. 

 The heat of combustion is sometimes given in British Thermal 

 Units per pound. The molecular weight of a gas is 28*87d, 

 and since there are 252 small calories in one B.T.U. and 

 454 grams to the pound, 



n = B.T.U. per lb. x 252 x 28*87 x d/53 x 454 x 10 3 . 



= 3-025 x B.T.U. xdxlO- 4 . 



Taking, for example, the petrol mixtures examined by 

 Thomas and Watson, quoted by Clerk and Burls *, we 

 obtain the values of n in Table II., and the percentage 

 mixtures in air giving this, the ratios of the volumes of air 

 and vapour, and of their weights. Since the substance 

 consists mostly of paraffins n will be in general a whole 

 number. In order to bring Thomas and Watson's heats of 

 combination into line with those for the paraffins, which 

 have been closely determined, they have been multiplied 

 by 1-13. 



Table II. 



Vapour Per cent. Vols, of height 



Calorific density of gas in air to ^ ^ 



n,-,™™ l value. at 0° 0. W<3al - n air for one of — — - 



Compound. BTJJ ^ irom ^^ perfeot inflam . weight 



per lb. 760 mm. formula. com . ma ble ot 



Air=l. bustion. vapour, vapour. 



Methane 23,800 0-554 3"98 4 9"35 97 17'5 



Ethane 21,500 1-074 7 7 5-5 17 '2 16'0 



Butane 21,300 201 12*95 13 3-07 31'5 15'6 



Pentane 21,200 251 1606 16 251 388 15*45 



Hexane 21,200 2-99 19-15' 19 2-13 46*0 15-4 



Heptane 21,150 3-45 22*1 22 1-84 53'3 15-45 



taken 



Bowley's 21,600 3'05 20 20 202 48'5 15-9 



Carless 21,200 3-11 20 20 2'02 48*5 15'6 



Express 20,400 3'35 21*2 21 1'92 51'2 15-26 



Ross 21,100 3-33 21-4 21 192 51*2 15-35 



Pratt (a) 21,030 3'16 20-2 20 2-02 48-5 15-35 



Pratt (b) 20,090 3-20 20-6 21 192 512 16-0 



Carburine 21,120 3"28 21/9 22 1'84 53 -3 16-28 



Shell ordinary. 21,150 3'27 219 22 1*84 533 16-32 



Dynol 20,930 3'43 217 22 1'84 53-3 16-55 



Simcar benzol. 19,300 3"24 18'9 19 2'13 46 14*2 



0760 (Baillie). 20,950 3'29 21 "4 21 T92 51-2 15*6 



0-760 Shell ... 20,620 3"36 218 22 1-84 53-3 15-9 



Coaline 18,850 3'31 18"9 19 213 46 13'9 



Mean values... 20,700 3-26 207 207 1"95 5()-4 15'5 



* ' The Gas, Petrol, and Oil Engine/ vol. ii. p. 452. 



