251. Energy emitted by a radio-active product. An important consequence follows from the fact that the heat emission is a measure of the energy of the expelled α particles. If each atom of each product emits α particles, the total emission of energy from 1 gram of the product can at once be determined. The α particles from the different products are projected with about the same velocity, and consequently carry off about the same amount of energy. Now it has been shown that the energy of each α particle expelled from radium is about 5·9 × 10^{-6} ergs. Most of the products probably have an atomic weight in the neighbourhood of 200. Since there are 3·6 × 10^{19} molecules in one cubic centimetre of hydrogen, it can easily be calculated that there are about 3·6 × 10^{21} atoms in one gram of the product.
If each atom of the product expels one α particle, the total energy emitted from 1 gram of the matter is about 2 × 10^{16} ergs or 8 × 10^8 gram calories. The total emission of energy from a product which emits only β rays is probably about one-hundredth of the above amount.
In this case we have only considered the energy emitted from a single product independently of the successive products which may arise from it. Radium, for example, may be considered a radio-active product which slowly breaks up and gives rise to four subsequent α ray products. The total heat emission from one gram of radium and products is thus about five times the above amount, or 4 × 10^9 gram calories.
The total emission of energy from radium is discussed later in section 266 from a slightly different point of view.
252. Number of ions produced by an α particle. In
the first edition of this book it was calculated by several independent
methods that 1 gram of radium emitted about 10^{11} α
particles per second. Since the actual number has later been
determined by measuring the charge carried by the α rays
(section 93) we can, conversely, use this number to determine with
more certainty some of the constants whose values were assumed
in the original calculation.
For example, the total number of ions produced by an α
