The radioactive elements did not escape this new view either. The atomic weight of uranium (chemical symbol U) is just about 238 and, indeed, most of its atoms are ²³⁸U. In 1935, however, the Canadian-American physicist, Arthur Jeffrey Dempster (1886-1950), found that 0.7% of its atoms were a lighter isotope, ²³⁵U. These differed considerably in radioactive properties. The common uranium isotope, ²³⁸U, had a half-life of 4500 million years, while ²³⁵U had a half-life of only 700 million years. Furthermore ²³⁵U broke down in three stages to actinium. It was ²³⁵U, not actinium itself, that was the beginning of the actinium radioactive series. As for thorium (atomic symbol Th) with an atomic weight of 232, it did indeed turn out that in the naturally occurring element virtually all the atoms were ²³²Th. ENERGY The Law of Conservation of Energy We have now gone as far as we conveniently can in considering the intertwining strands of the atom and of electricity. It is time to turn to the third strand—energy. To physicists the concept of “work” is that of exerting a force on a body and making it move through some distance. To lift a weight against the pull of gravity is work. To drive a nail into wood against the friction of its fibers is work. Anything capable of performing work is said to possess “energy” from Greek words meaning “work within”. There are various forms of energy. Any moving mass possesses energy by virtue of its motion. That is, a moving hammer will drive a nail into wood, while the same hammer held motionlessly against the nailhead will not do so. Heat is a form of energy, since it will expand steam that will force wheels into motion that can then do work. Electricity, magnetism, sound, and light can be made to perform work and are forms of energy. The forms of energy are so many and so various that scientists were eager to find some rule that covered them all and would therefore serve as a unifying bond. It did not seem impossible that such a rule might exist, since one had been found in connection with matter that appeared in even greater variety than energy did. All matter, whatever its form and shape, possessed mass, and in the 1770s, the French chemist Antoine Laurent Lavoisier (1743-1794) discovered that the quantity of mass was constant. If a system of matter were isolated and made to undergo complicated chemical reactions, everything about it might change, but not its mass. A solid might turn into a gas; a single substance might change into two or three different substances, but whatever happened, the total mass at the end was exactly the same (as nearly as chemists could tell) as at the beginning. None was either created or destroyed, however, the nature of the matter might change. This was called the “law of conservation of mass”. [Illustration: _Lavoisier in his laboratory during his studies on respiration. From a sketch made by Madame Lavoisier._]