The determination of the specific heats of gases and vapours is attended with difficulty. Itegnault's results. are probably very near the truth. Taking the specific heat of an equal weight of water as the unit of comparison, be finds that of air to be =0'2377. The following are a few of his results mate object of convex spectacles is to restore the natural power (or rather tho natural focus) of the eyo, and not to enable the wearer to bee objects larger or more distinctly than with the eyo in a strong and healthy condition. Short-sightedness being still leas dependent upon ago, cannot be met by any rule even so well as the opposite defect.
In choosing spectacles, the frame should fit comfortably to the head, and be of such a form as to bring tho centre of each lens exactly oppo site to the centre of the eye. Should this not bo the case, the bridge or wire that unites the two halves of the frame may be btretclicd or bent to suit the wearer.
With respect to the glass used in spectacles, it may be taken as a rule that, except in cases where it is necessary to protect the eyes from an Injurious glare of light, the most colourless material is to be preferred. The accurate figure of the lenses cannot be too strongly insisted on, and may bo tested by holding the spectacles near to a printed book, and gradually moving them nearer to the eye ; by doing which, if the glasses be not well-shaped, tho letters will appear distorted. Veins or blebs in the glass are injurious, and may be detected by holding the glass between the eye and the flame of a candle, and moving it back wards and forwards, until it appears full of light ; when every such defect will be distinctly seen. Scratches aro not quite so injurious,
inasmuch as they do not produce distortion ; yet they should be avoided. Their diminished liability to injury by scratching forms the principal recommendation of spectacle lenses of rock crystal ; but some opticians consider their use injurious, owing to their tendency to irregular refraction of the revs of light.
Several deviations from the ordinary mode of constructing specta cles may be alluded to. The 'periscopic spectacles of Dr. 11'ollaston were contrived in order to allow considerable latitude of motion to tbo eyes without fatigue, by conforming the shape of the glasses to that of the eyes. This is effected by tho use of lenses either of a meniscus or coneavo-convex form : the concave side being in both cases turned towards the eye. Fig. 1 represents in section, the form of the lens There is a remarkable relation between the specific heat of an ele mentary body and its chemical equivalent. If tho specific heat of a body in the solid state be multiplied into the chemical equivalent of the name body, it gives a number which coincides almost exactly with the product obtained by multiplying together tho specific beat and the equivalent of any other elementary substance.