ON PETROLEUM AND PHOTOUEN. 31 



least development of heat. The following tables are by Dr. Frank- 

 land ; the first gives the illuminating equivalents of various 

 materials, showing the quantity of other substances which 

 would be required to give the same amount of light as would be 

 obtained from one gallon of Young's Paraffin oil. 



Young's Paraffin oil .... TOO gallons. 



American Eock oil .... 1"26 „ 



Paraffin candles 18-6 pounds 



Sperm „ 22-9 „ 



Wax „ 26-4 „ 



Stearine „ 27'6 ,, 



Composite,, 29'5 „ 



Tallow „ 39-0 „ 



The following table shows the comparative cost of the light of 20 

 sperm candles, each burning 10 hours at the rate of 120 grains per 

 hour, and of the heat evolved per hour : — 



s. d. Unit of heat. 

 Tallow . . . . . .28 100 



Wax 7 2£ ) 



Sperm 6 8 ( tiZ 



Paraffin candles . . , . 3 10 66 



Rock oil 7 I 9 q 



Paraffin oil 6 S 



Coal gas ..... 4J 47 



Cannel gas .... 3 32 



The objections which have operated chiefly against these oils as 

 agents for the production of artificial light have arisen from the fear 

 of explosion, and the unpleasant odour produced during their com- 

 bustion. It is unquestionable that accidents have occasionally arisen 

 from explosion of the oil in lamps by the ignition of the explosive 

 mixture which their vapours form with air. When the accident occurs 

 it arises from imperfect purification of the liquid, by the imperfect 

 removal of the benzine or more volatile constituents, so that the safety 

 of a given sample depends upon the temperature at which vapour is 

 given off. The purification is now more efficiently performed than 

 when the materials were first manufactured, and as the cistern of a lamp, 

 especially if made of glass or other nonconducting substance, is never 

 likely to rise higher in temperature than 100 deg., all oils which do not 

 give off vapour at that heat may be considered safe. A simple test is the 

 application of a light to a small quantity, if it takes fire, and burns 

 like alcohol it is unsafe. The other objection is, the odour which they 

 produce during combustion, various oils differ in this respect, which in 

 some measure depends upon their constitution and their purity ; but it 

 may be greatly lessened or entirely overcome by attention to certain 

 precautions. The chief causes are imperfect combustion with conse- 

 quent production of offensive empyreumatic substances, and the volatili- 

 sation of a portion of the fluid unconsumed. The lamp should 

 therefore be burnt with a good flame, and the wick well turned up, the 

 cistern of the lamp not filled too full, and all the metal of the burning 

 parts kept scrupulously clean. 



The length of this notice of petroleum precludes the consideration 

 of the uses of a large number of other substances which are obtained with 

 the oil by the distillation oi coal, and which admit of most valuable 

 and extensive employment such as Benzole, Paraffin, &c, as well as the 

 application of Rock oil to the manufacture of illuminating gas which 

 has been carried out successfully in New York. 



