IN STUDYING QUESTIONS OF CHEMICAL MECHANICS. 309 
other more sensible applications, and this I shall do in what 
follows. 
12. The permanence, at least very approximate, of constitu- 
tion which we have just seen in the molecules of the essence of 
turpentine when their mutual distances are increased by a mode- 
rate elevation of temperature, leads us to conceive how they still 
preserve an optical power of the same nature and of the same 
direction when separated from one another so far as to reduce 
them to the state of elastic vapour in motion. The only expe- 
riment of this kind which has been attempted, and which was 
unfortunately incomplete, was made by transmitting the vapour 
of the essence in a continuous current through a metallic tube 
15 metres long, inclosed in another tube in which it first cir- 
culated, in order to maintain in it an external temperature suffi- 
ciently elevated. After all the air was expelled, the column 
of vapour in motion was observed to impress on the polarized 
light a very perceptible deviation in the same direction as that 
of the liquid, that is to say directed also towards the left, and 
producing in the same manner coloured images of variable tints, 
which succeeded one another in a similar order, when the prin- 
cipal section of the analysing prism was progressively brought 
into different azimuths around the primitive plane of polariza- 
tion. Two observers, who assisted me in this experiment, were 
witnesses of these facts. Unfortunately an explosion which oc- 
curred in the apparatus prevented my profiting by the arrange- 
ments which I had prepared to measure the temperature of the 
current and its elastic force when it should have arrived at a 
fixed state, which would have enabled me to calculate the density 
of the vapour and to compare the relative energy of the molecular 
conditions in two such different states of the same body. If this 
experiment were repeated and extended to various active essen- 
tial oils, it would doubtless furnish extremely interesting results, 
especially on comparing those which would be obtained as well 
on such of these bodies as might undergo a certain degree of 
vaporization without decomposition, as also for such as this 
operation, repeated or continued for some length of time, would 
progressively aiter. 
13. The individuality of power, however, which the active 
groups preserve in this state of indefinite separation, induced by 
vaporization, indicates a much more simple and convenient 
means of varying their intervals without altering their influence 
