Practical men are accustomed to measure the efficiency of a glow-lamp in watts per candle or, in other words, by the quantity of energy which must be supplied to it for every mean spherical candle-power of light it gives. The energy supplied is measured by an ammeter and voltmeter, and the mean spherical candle-power is obtained on the photometric bench by comparing the light given in different directions with that of some standard source. The watts per candle together with the life and initial cost of the lamp determine its commercial value.

On the other hand, we may regard the lamp as an energy-transforming device, and measure its efficiency by the percentage of electrical energy it transforms into light.

In a communication made to this Society on December 6, 1897, a tendency towards maximum specific electrolytic conductivity was shown to be a characteristic of many chemical reactions. This tendency is very marked in reactions in which strong acids play a part. Thus, to take a well-known example, chlorine decomposes water under the influence of light with production of hydrogen chloride and oxygen, but as the hydrogen chloride accumulates in the solution, the decomposition of the water is more and more retarded. This photo-chemical reaction is a reversible one, for in the case of highly concentrated solutions the hydrogen chloride is oxidised to water and free chlorine.

Daniel John Cunningham was born at Crieff on April 15, 1850. He was a son of the manse, and his father, the Rev. John Cunningham, subsequently became Principal of St Mary's College, St Andrews University. His mother was descended from a brother of Captain Porteous, of the Porteous Riot fame.

Daniel Cunningham was first educated at a small private school started by the heads of six families, who engaged a teacher for their boys. Ere long, however, Crieff Academy was organised, and to it the younger scholars were transferred. Cunningham received the simple, strong education of which Scotland was proud in those days, an education which fitted a promising boy for any walk in life. His first intention was to enter on a business career, and with this object in view he spent a year or two in Glasgow.