OP ARTS AND SCIENCES. 63 



mediate small fractions lying between the prominent ones were 

 repeatedly and so thoroughly worked over as to prove the absence 

 of other bodies than those described, i. e. in quantities appreciable 

 by fractional condensation. I do not, however, ignore the fact 

 that small quantities of other bodies might be present and escape 

 detection. That any appreciable constituent of that part of the 

 petroleum worked upon was overlooked in this investigation is the 

 more improbable from the fact that, in consequence of the dis- 

 covery of the new series isomeric with the hydrides, vastly more 

 time and labor were given to these separations than are ordinarity 

 required, or than would have been necessary otherwise. The ex- 

 tremely small difference of boiling-point between the correspond- 

 ing or opposite members of these two parallel series has necessitated 

 a most thorough and critical examination, requiring a vast number 

 of redistillations, each extending over the whole range of frac- 

 tions, — a fraction being taken for every degree of temperature, — 

 and this extremely tedious process was continued until the small 

 intermediate fractions, as before stated, had become too small to 

 admit of being further operated upon; and until the boiling tem- 

 perature of the large fractions herein treated of had ceased to un- 

 dergo further change by this treatment. This course was deemed 

 indispensable, — regardless of the large expenditure of time in- 

 volved, — since nothing less could suffice to remove all doubt as 

 to the existence of the newly discovered series, and insure for the 

 different substances that degree of purity requisite for any satis- 

 factory study of their individual properties. 



I have laid special stress on the circumstance here related, since 

 it appears, on comparison of my results with those of Pelouze and 

 Cahours, that they claim to have found one more body between 

 about 120° and 236° than I have been able to discover. Either I 

 have overlooked in my anal} r sis one constituent of the petroleum 

 between these limits of temperature, or they have described one 

 having no existence therein; in this connection, for obvious rea- 

 sons, my newly discovered series of isohydrides is of course not 

 taken into account. To facilitate a comparison of their results 

 with my own, as the latter are exhibited in the preceding tables, I 

 will here introduce the following table prepared by me for this 

 purpose.* 



* At this date the nomenclature here employed may appear antiquated ; 

 but since this does not affect the intrinsic value of the paper, I have preferred 

 to let it pass through the press as written several years ago. 



