148 Extracts from the Journals. [Jan., 



abundance of alumina and of insoluble silicates in the latter. The 

 matter soluble in water is nearly the same for both, and the oxide 

 of iron not widely diflerent. 



GAS ILLUMINATION. 



The only method of strict accuracy by which the value of an il- 

 luminating gas maybe determined, is by a complete chemical analy- 

 sis; but this is an operation of extreme delicacy, requiring far more 

 adroitness in minute manipulation than is possessed by the gene- 

 rality of gas engineers. Results, however, of sufficient accuracy 

 for all ordinary purposes, may be readily obtained by methods 

 much more easily executed; namely, P, by a photometrical ex- 

 periment; 2°, by determining the specific gravity of the gas; and 

 3°, by determining the quantity of oxygen required for the com- 

 plete combustion of the gas, with the amount of carbonic acid 

 produced. The first of these methods is the simplest; and the re- 

 sults it affords, when performed with care, are equal in value to 

 those got by the two other methods. 



The photometrical processes which I shall briefly describe are 

 founded upon principles of extreme simplicity. Though the eye 

 is unable to judge with precision of the relative intensity of two 

 lights, yet it can determine with considerable accuracy when con- 

 tiguous shadows of an opaque object thrown upon a screen by 

 different lights are equally dark, or when two similar adjoining 

 surfaces are equally illumined, provided the lights are of the same 

 teint. If the two lights which produce these effects are equal in 

 intensity, obviously their distance from the screen must also be 

 equal; but if unequal, the most intense light is placed farthest 

 from the screen. Now, as the rays of light are propagated con- 

 tinually in straight divergent lines, their intensity diminishes in the 

 direct proportion of the square of their distance from their source. 

 Taking, as a standard, the amount of light on a screen derived 

 from a flame at the distance of one foot, then at two feet the light 

 on the screen from the same source would be one-fourth, at three 

 feet one-ninth, and at four feet one-sixteenth of the standard. 

 Therefore, if two or more sources of light are so placed as to cast 

 an equal light on the screen, their relative intensities are directly 

 as the square of their distances from the screen. The objection 

 to this mode is, that it does not readily admit of a fixed standard 

 of comparison. 



The method of contrasting the shadow of an opaque object 

 formed by different lights was first employed by Lambert (Photo- 



