230 Native Bitumens and the Pitch Lake of Trinidad. [April, 
In their structures, the Romans directed much attention to 
solidity and permanence, and of course endeavored to select 
what were considered the most useful and durable materials. 
That these materials were often good is shown by the state of 
preservation of many of their works, and by the fact that their 
cement is scarcely equaled by any of modern time; and yet 
Vitruvius, a celebrated architect of the age of Augustus, speaks 
of bitumen as superior to every other kind of cement, and regrets 
its scarcity. 
Notwithstanding the long time that native bitumens have been 
known, it is only within the present century that they have come 
to be extensively employed in the arts; and that geologists and 
chemists have reached definite conclusions concerning their origin, 
modes of occurrence, properties and relations. The prevalent 
notion that these substances are of rare and limited occurrence is 
entirely erroneous, for, as I shall presently show, the bitumens, 
taken as a class, are very widely and abundantly diffused through 
the crust of the earth. They are found in every quarter of the 
globe, and in every geological formation from the Cambrian to 
the present time. Their occasional association with what appear - 
to be igneous rocks, has led some writers to infer that in their 
origin they are in some way connected with volcanic action. An 
explanation which, as Canon Kingsley has remarked, “ savors 
somewhat of a ‘bull;’ for what a volcano could do to pitch, save 
to burn it up into coke and gases, it is difficult to see.” When, 
as undoubtedly sometimes happens, the bore of a volcano passes 
through sedimentary strata holding bitumen or bituminous coal, 
it is easy to see how the connection of these substances with 
volcanic products may arise. But be their associations what they 
may, it has been definitely settled that in their origin the bitu- 
mens, like the coals, are always strictly organic. In every case 
they are the more or less transformed tissues of plants or 
animals. 
Under the general name of bitumen are included both the 
liquid forms, petroleum and naphtha, and the solid varieties such 
as asphalt. Chemically considered, the bitumens are hydrocarbons 
the average composition being represented by the general 
formula C, Ha. The so-called bituminous coals, which, how- 
ever, are destitute of true bitumen, are likewise hydrocarbons. 
These are distinguished from the bitumens by their smaller 
hydrogen ratio, analysis affording the general formula C, Han and 
