552 TRANSACTIONS OF THE 



by judicious and close study ot their physical properties, by ob- 

 serving the effects produced on them by certain re-actives or by 

 comparatives, by good typical specimens that we can arrive at 

 their degrees of priority. 



Among the physical properties of oils, we particularly insist on 

 their specific gravities. But as these differences are very minute, 

 it is essential to have an instrument of very great sensibility to 

 measure them. The hydrostatic balance has been rejected, and 

 unless we can have one possessing the precision and sensibility of 

 the balances used by chemists in their quantitive analyseo, and 

 an observer able and experienced, we shall commit serious mis- 

 takes. 



Mons. Lefebvre, of Amiens, has furnished a cold oleometer. 



It resembles the areometer, (measures density of fluids,) only 

 that the graduation is so calculated that the specific gravities of 

 the oils are read on the scale, as they are at 15 degrees Centigrade 

 =57 degrees Fahrenheit, as unity. 



The specific gravities of fixed oils are inferior to w^ater, but, 

 with one single exception, are nearly all comprised within 0.900 

 and 1.000. Hence it would be useless to figure the first decimal 

 on the oleometev scale. For example : Suppose the level of 

 liquid to be at division 34, we must conclude that the specific 

 gravity is 0.934, at the temperature of the period of observation. 



A description of the instrument follows : 



The fixed oils all possess a more or less active affinity for oxy- 

 gen, which they absorb when exposed to the air by disengaging 

 the carbonic acid. They then begin to thicken, and at last 

 finish by becoming solid, so as not even to soil paper. This result 

 occurs rapidly when the oil is spread thin; it becomes, as it were, 

 resinified, and represents a hard, solid varnish. This absorption 

 of oxygen is accompanied by disengagement of heat, like all 

 chemical reactions, and in certain circumstances may become in- 

 tense enough to give spontaneous inflammation to bodies oiled. 

 Terrible conflagrations have been caused by it, especially in spin- 

 ning mills. Rags used to clean oiled machinery, &c., never should 

 be laid by in heaps. Some oils always retain their unctuous 

 qualities, yet thicken by exposure to the air. 



Hence the distinction hi^ivfeen fixed oils and drying oi\?>. Fixed 

 oils are extracted from both animal and vegetable bodies. Ani- 

 mal from the ftit, vegetable chiefly from the seeds. Here follow 

 descriptions of vegetable oils, such as linseed, colza, olive, &c. 

 Olive oil is the least dry of oils. Virgin olive oil has a fine, 



