TRANSACTIONS. 
73 
the separating surface, and its colour will undergo corre¬ 
sponding changes. 
The principal part of the instrument is a triangular prism 
of gl ass, between the lower surface of which and the upper 
surface of the mineral the oil is placed. This oil will form a 
parallel film, but, by the mechanism of the instrument, the 
two surfaces which bound this film can be inclined to each 
other, so that an eye looking into the prism will see at once 
the images of a luminous body, such as the sun, &c., reflected 
from the separating surface of the oil and the prism, and from 
that of the oil and the mineral. The first of these images is 
constant both in its colour and quantity of light, while the oil 
is the same, but the second will vary with the mineral. The 
comparison of the colour and quantity of light obtained from 
different minerals furnishes the nicest tests for discriminating 
them. 
The author illustrated his explanations of the principle of 
the instrument by means of diagrams, and the instrument itself, 
as constructed by Dollond, w r as exhibited to the Meeting. 
THURSDAY MORNING. 
Mr. Dalton read a paper written for the Literary and Philo¬ 
sophical Society of Manchester, containing a series of experi¬ 
ments on the quantity of food, taken by a person in health, 
compared with the quantity of the different secretions; with 
chemical remarks and deductions. 
Mr. Dalton, whose regular habits of life and uniform good 
health enabled him to make these Experiments upon himself to 
great advantage, commenced them about 40 years since at Ken¬ 
dal, and prosecuted them for periods of a week or a fortnight at 
various seasons of the year, to ascertain the proportion between 
the weight of food, and the ordinary evacuations. Particular 
observations were made on the effects occasioned by drinking an 
infusion of Carbonate of Potash, and a train of experiments 
was continued for three weeks to determine the loss of weight 
by perspiration for the whole day, and for certain hours of 
the morning, afternoon, and night. The mean daily loss by 
perspiration was 37-| oz. 
From these experiments, the state of organic chemistry 40 
years since did not permit Mr. Dalton to make any deductions, 
but he was now enabled to return to the subject with the 
powerful aid of exact analysis. He showed that the quantity 
of Carbon contained in the solid and liquid food taken into the 
