The Romance of Modern Chemistry

INVISIBLE SUBSTANCES forming what may be regarded as invisible pools. Hence it has occasionally happened that a brewery worker, de- scending into one of the large vats for the purpose of cleaning it, has collapsed fatally—practically drowned in the carbon dioxide which had collected at the bottom of the vat. When a descent has to be made either into a brewer’s vat, or into an old well, where a similar accumu- lation of carbon dioxide may occur, a lighted candle ought first to be lowered to the bottom. Should the candle continue to burn as brightly as in the open air, no one need hesitate to follow it If the candle, however, goes out, or even gets dim only, it is evidence that there is a dangerously large quantity of carbon dioxide present. The element carbon combines with oxygen in more than one proportion, giving rise not only to carbon dioxide, but also to carbon monoxide. This latter substance is a colourless and odourless gas, which burns with a blue flame and is intensely poisonous. Any one who watches a clear coal fire on a winter evening will notice little tongues of blue flame; these are due to carbon monoxide, which readily combines with more oxygen to form carbon dioxide. Carbon monoxide has a curious effect on the blood—an effect which is directly associated with its poisonous properties. It has the power of forming a com- pound with the haemoglobin, the colouring matter of the blood, and this involves a slight change of tint. By shaking up the suspected gas with a little blood, and then comparing the latter with some of the original blood, either by mere inspection or by means of a spectroscope, one may detect quite small quantities of carbon monoxide. Some very interesting cases are on record in which mice have been used to indicate the presence of carbon mon- oxide in an atmosphere. Small animals, such as mice, are 45