was the real nature of the relationship between heat and mechanical power critically examined, save once in a quickly forgotten investigation by Sadi Carnot. But at length the speculations and calculations of Julius Robert Mayer, the admirable experimental researches of Joule, and the profound studies of Helmholtz and others established the principle of the conservation of energy[1]—in short, demonstrated the proposition that energy is one and indestructible, however it may manifest itself as heat, or light, or electricity, or otherwise.
2. Periodicity
Somewhat later the work of Newlands, Lother Meyer, and Mendeléeff brought to light an extraordinary series of relationships, periodically recurring properties, among the elements. It would be impossible briefly to explain this relationship, but a simple analogy may serve to show its nature.
| 11 | 12 | 13 | 14 | 15 |
| 21 | 22 | 23 | 24 | 25 |
| 31 | 33 | 34 | 35 | |
| 41 | 42 | 43 | 45 | |
| 51 | 52 | 53 | 54 | 55 |
Giving the numbers above arranged, there can be no doubt, first, that they have been correctly arranged, and secondly, that the numbers 32 and 44 are missing, but have a place in the table. In other words, it is possible to predict the "properties" of the two missing numbers. In like manner, the studies of Mendeléeff showed similar connections among the elements. These could be arranged, as he showed, in the order of their atomic weights, in a table very similar to the above, in which the variation in properties was regular and periodically recurrent, but with certain gaps in the classification. Judging from the elements surrounding such gaps, Mendeléeff predicted the properties of the missing elements in certain cases in which the missing elements have now been supplied by chemical research. The results have invariably confirmed the Russian chemist's predictions, as may be seen from the following data concerning the element germanium:
- ↑ H. C., xxx, 173 ff.
