OP ARTS AND SCIENCES. 69 



2. Amyl Hydride = C^H 



12- 



Specific gravity 0.6400 at 0°; 0.6275 at 14°. 2; 0.6269 at 15°; 

 0.6249 at 17°.* 



Determination of Boiling-point. — The method employed in this, 

 and in all of the determinations of hoiling-points given in this 

 paper, unless some variation is specially mentioned, was precisely 

 the same as that described in detail in the memoir first above re- 

 ferred to, page 159, and following. As constancy of boiling-point 

 and accuracy of determination of the same are of so much impor- 

 tance in this investigation, and as I have deviated in some respects 

 from the usual method of taking boiling-points, attention is spe- 

 cially called to what is said in that memoir on this subject; more 

 particularly to the remarks on the question as to whether the ex- 

 periment should be conducted with the bulb of the thermometer in 

 the liquid or in the vapor. Some experiments there recorded bear- 

 ing on this question are also referred to. 



In order to show more fully the degree of purity of the sub- 

 stances, and the degree of thoroughness and accuracy of the analy- 

 sis of the petroleum which this may indicate, I shall give in each 

 case the readings of the thermometer at stated intervals during the 

 distillation either of the whole, or very nearly the whole, of the 

 liquid employed in the experiment, taking a fair average of these 

 as the observed boiling-point. This, it will be observed, is a much 

 severer test of purity, and is attended with less liability to per- 

 sonal error, than the more common practice of simply noting the 

 temperature at which a certain degree of constancy of boiling-point 

 is observed, omitting to indicate the limits of the distilling tem- 

 perature. The custom here referred to has the additional objec- 

 tion, moreover, that it is liable to convey a wrong impression as to 

 the actual degree of purity of the liquid, — not giving sufficient 

 data to enable others to judge of this and of the accuracy of the 



* It being too generally the custom, as I have remarked on a former occa- 

 sion, to determine the specific gravities of volatile liquids at the temperature 

 most convenient at the time of the experiment, without regard to uniformity 

 of practice, which is so highly desirable, I have been obliged, in order to make 

 a satisfactory comparison of other determinations with my own, to repeat the 

 determinations at the temperatures employed by the different observers. As 

 the results of these determinations occupy so little space, I am induced to insert 

 them, as they were all taken with great care, and by means of a specific gravity 

 bottle of the best construction for volatile liquids. A description of this bottle 

 is given in Vol. IX. (N. S.) p. 144, of the Memoirs of this Academy. 



