Many fatal accidents have occurred in consequence of the air in deep pits, graves, &c., becoming unfit for respiration by the accumulation of carbonic acid gas, which may arise either from cavities in the soil, where animal matter has undergone decomposition, or it may happen from the depth and narrowness of the hole or well preventing a proper draught or current of air, so that it becomes foul by the breathing of the man who is digging the pit. Air which contains one or two per cent. of carbonic acid will support the respiration of man, or maintain the flame of a candle; but it produces the most serious results if inhaled for any length of time; a lighted candle let down into a well (suspected to contain foul air) before the descent of the person who is to work in it, may burn, but does not indicate the presence of the small percentage of the poison, carbonic acid. Frequently no trouble is taken to test the air with a lighted candle; a man is lowered by his companions, who see him suddenly become insensible, another is then lowered quickly to rescue him, and he shares the same fate; and indeed cases have occurred where even a third and a fourth have blindly and ignorantly rushed to their death in the humane attempt to rescue their fellow creatures. What is to be done in these cases? Are the living to remain idle whilst the unfortunate man is suffocating rapidly at the bottom of the pit? No; provided they do not venture themselves into the pit, they may try every known expedient to alter the condition of the foul air, so as to enable them to descend to the rescue. One should be despatched to any neighbouring house or cottage for a pan of burning coals; if any slacked lime is to be had, it may be rapidly mixed with water, and poured down the side of the pit; a bundle of shavings set on fire and let down, keeping it to one side, so as to establish a current; or even the empty buckets constantly let down empty and pulled up full of the noxious air, may appear a somewhat absurd step to take, but under the circumstances any plan that will change the air sufficiently to enable another person to descend must be adopted; in proof of which the following experiments may be adduced: