In both, it will be noticed, the result is a single positive. It is not, as in ordinary photography, a negative off which any desired number of positive prints can be made. And, moreover, it is a transparency: it cannot be viewed except by light shining through it. The results are, however, extremely beautiful, when well done, and anyone who cares to try either of these methods of working will be well repaid for the trouble involved. CHAPTER XVII HOW SCIENCE AIDS THE STRICKEN COLLIER Nothing is more characteristic of the present age than the care which is, quite rightly, expended upon the comfort and safety of those who do the manual labour of the community. The stores of scientific knowledge and skill are drawn upon freely for this end, and some very interesting examples can be given of the truly scientific methods which have been evolved, not only for preventing injuries of any kind, but for succouring those who may, despite those precautions, fall victims to disease or accident. An example has already been given of the scientific investigation into the nature of colliery explosions and the best means of preventing them. We have seen there how expense has been poured out lavishly in fitting up the experimental gallery or artificial pit, and how the most cunning mechanical and electrical devices have been pressed into the service in order to find out just what happens when an explosion occurs. It has been related how these investigations have revealed with certainty the true cause of the explosions and thereby led the way to their prevention. But with it all there is still an occasional disaster, occurring, sometimes, at the best and most carefully managed collieries. And therefore it is still necessary to provide for rescuing the unfortunate men who are affected. It is worth remark, here, that colliery explosions are, all things considered, a very rare occurrence. Because of their dramatic suddenness, and the number of lives which are commonly lost in a single disaster, we are apt to magnify their severity in our minds and to picture the life of the miner as a very hazardous one. In point of fact, the expectation of life, as the insurance people call it, is quite as great among the coal-miners as among any class of manual labour. And of those who do meet an untimely end there are more lost through isolated accidents, involving one or two men, than in the great disasters.