646 THE MODE OF TESTING MINERAL OILS USED FOR LAMPS. 
some allowance requires to be made for it; and since, in this respect, the degree of in¬ 
flammability is the character of the greatest importance, it is customary, in the opera¬ 
tion of refining, to separate and exclude from the oil to be used for ordinary lamps, a 
certain amount of the more volatile hydrocarbons contained in the crude petroleum or 
paraffin oil. The extent to which this is done is not in all cases the same, and there is 
some difference of opinion as to what should be the minimum degree of inflammability 
of the oil, or the minimum temperature at which it should be capable of giving off in¬ 
flammable vapour, in order to ensure for it such a degree of safety as would be consistent 
with the general circumstances under which it is used in lamps and kept as a commodity 
in shops. By some it is considered that when a flame is brought into actual contact with 
the oil, heated in an open capsule, it ought not to take fire, so as to continue burning, 
until the temperature reaches 130° F. Others, again, consider that, if it does not take 
fire at a temperature below 100° F., it is of a character fit to meet all requirements. 
Hitherto, far less attention has been paid to the degree of inflammability of mineral 
oil than to the absence of smell and colour. It has also been customary in different 
countries to use oil differing widely in regard to the temperature at -which it would take 
fire. Some years ago, the oil generally consumed in this country had a very high firing- 
point, and it was customary to test it by observing how long it would bear contact with 
the flame of a match without taking fire, or more accurately by observing the tempe¬ 
rature at which it took fire and continued to burn. This temperature was generally 
from 120° to 130° F., and sometimes it was as high as 150° F. The oils manufactured 
about eight years ago, from Rangoon petroleum, and from coal or bituminous shale, were 
of this kind. On the Continent, oil inflammable at a much lower temperature has gene¬ 
rally been preferred. 
After the introduction of American petroleum, there was a marked difference in the 
character of the imported lamp-oil manufactured from this material, so far as regards the 
degree of inflammability, and it would often take fire at a temperature little above 100° 
F. During the last three years, the firing-point of this oil has been still further reduced, 
and probably the greater part of the lamp-oil imported from America and made from 
petroleum, which has been consumed during that period, has had a firing-point below 
100° F., sometimes considerably lower. 
In addition to the differences, both of opinion and practice, in regard to the degree of 
inflammability of mineral lamp-oil, there are also similar differences as to the mode of 
determining this character, and as to what is to be regarded as the firing-point. 
This oil being in all cases a mixture of several hydrocarbons, belonging to the paraffin 
series chiefly, and varying in their boiling-points from about 200° to upwards of 700° F., 
the temperature at which it begins to give off vapour will depend upon the boiling-point 
of the more volatile hydrocarbons, and upon the amount of them contained in the oil. 
Consequently, when the oil is gradually heated in contact with air, and a flame is brought 
near the surface from time to time, there is always a little momentary flash, produced 
by the vapour taking fire; and this happens some time before the oil attains the tempe¬ 
rature at which it takes fire itself, and continues to burn. The difference between the 
temperature at which the temporary flash takes place and that at which the oil takes 
fire, will amount to from 10 to 20° F. 
Upon the assumption that an explosive mixture of oil-vapour and air may be formed 
within the reservoir of a lamp, it has been urged that the temperature of the oil when 
this temporary flash takes place should be considered as the firing-point of the oil. 
The quantity of vapour given off by mineral oil at the temperature when the first 
temporary flash takes place is generally very small, even when the oil itself takes fire at 
100° F.; and as the oil in the glass reservoir of a paraffin lamp, while it is burning, 
seldom acquires a temperature more than about ten degrees above that of the surrounding 
atmosphere, there would not be much probability of any such quantity of vapour being 
produced within the reservoir as could give rise to explosion by contact with flame. In 
regard to this point, however, the construction of the lamp is of considerable importance. 
In some lamps the pin by which the height of the wfick is adjusted is so arranged that 
there is free communication between the flame of the lamp and the oil reservoir, and 
between the two there is a small metal chamber, with a hole at the side, through which 
the pin passes. This chamber, in common with the other brass fitting of the lamp, being 
heated to a temperature much above 100° F., will of course become filled with vapour 
given off from the oil as it passes up through the wick; and that vapour, by mixing 
