The remarkable coolness of the electric light, as compared with its volume by gas, is also due in a great measure to the conspicuous absence of that large excess of less refrangible, or heat-radiating principle, which distinguishes almost equally all other modes of artificial illumination.  After the foregoing statement it may seem a paradox to claim that the electric arc develops the greatest heat with which we have yet had to deal, but this is so; and the heat has an intensity quite beyond the reach of accurate measurement by any instrument now known—­it has been variously estimated anywhere between 5,000 deg. and 50,000 deg.  F. It is sufficient for our present purpose to know that the most refractory substances quickly disappear when brought under its influence—­even the imperial diamond must succumb in a short time.  In order to reconcile this fact with its coolness as an illuminating agent, we have to take into consideration the extreme smallness of the point from which the light radiates in the electric arc.  A light having the power of many thousand candles will expose but a fraction of the surface for heat radiation which is shown by one gas-jet, and, as I have endeavored to explain, these rays contain very much less of the heating principle than those from gas or other artificial light.

The purity of electric light has another important aspect, which can scarcely be overestimated—­namely, the facility with which all the most delicate shades of color can be distinguished.  I understand from persons better skilled than myself in such matters that this can be done almost as readily by electric as by day light, and I have little doubt that the slight difference in this respect will entirely disappear when people become somewhat more familiar with the different conditions—­the effect of such shades viewed by electric light being more like that with comparatively feeble direct sunlight than the subdued daylight usually prevailing in stores and warehouses.

Again, it has frequently been urged that persons working by electric light have thus induced inflammation of the eyes.  No doubt this is so with light containing the highly refrangible rays in excess; but it is difficult to see how such an effect can occur with light composed as is the light with which the eyes are constructed to operate in perfect harmony.

As you are aware, there are other methods of obtaining light by electric energy, and in order to make a fair comparison of one which has lately attracted a great deal of attention and capital, I will relate to you the result of observations made during a recent visit to the office of an eminent electrician.  The light was that known as incandescent—­a filament of carbon raised to a light-emitting heat in vacuo.  The exclusion of the air is necessary to prevent the otherwise rapid destruction of the carbon by combination with oxygen.  At the time of my visit there were 62 lamps in circuit.  According