FOR the first of the series of Research and Development Lectures arranged under the auspices of the British Science Guild and delivered at the Royal Institution, Sir William Bragg, on May 2, took as his subject “Refrigeration”. This he pointed out is of great importance to Great Britain, which imports an immense amount of meat, fish, butter and fruit, many hundreds of shiploads of which are received every year. The principles underlying refrigeration are comparatively simple, but their application on a commercial scale has involved much research such as is being carried out at Cambridge, the National Physical Laboratory, East Mailing and elsewhere. Historically, the subject of heat and cold goes back to the early days of the Royal Society, and Hooke's views on fluidity are of much interest. In the eighteenth century, the theory of calorie held sway, but it was through the work of Rumford, Davy, Mayer and Joule that it was shown that heat is, in the phrase of Tyndall, a mode of motion, and to-day it can be shown that the molecules of substances are all in motion, the rapidity of which is increased by heat and decreased with cold. All the phenomena of expansion, compression and evaporation, which are utilised in refrigerating machines, are explained by this theory. Throughout the lecture, each step was illustrated by experiments in which billiard balls, bicycle pumps and liquid air played as important a part as thermo-couples and galvanometers. A singularly beautiful demonstration of the formation of vapour and clouds was given by pouring liquid air on to the surface of warm water lying in a large shallow pan. Liquid air was used also to show the alteration in the properties of substances when really cold, rubber becoming brittle and a bell of lead giving a metallic note when cooled in it. Sir William referred to the refrigeration exhibition now being held at the Science Museum, and on behalf of the director of the Museum invited all those in the audience to visit it.