By a variety of ingenious arrangements formic aldehyde has been tested by a large number of observers during the last two or three years. We may refer to three modes of application. 1. _The sprayer_ (Equifex apparatus) produces a mixture of air and solution for spraying walls, ceilings, floors, and sometimes garments. 2. _The autoclave_ (Trillat's apparatus). In this a mixture of a 30-40 per cent. watery solution of formaldehyde and calcium chloride (4-5 per cent.) is heated under a pressure of three or four atmospheres, and the almost pure, dry gas is conducted through a tube passing through the keyhole of the door into the sealed-up room. 3. _The paraform lamp_ (the Alformant). The principle of this lamp is that the hot, moist products from the combustion of methylated spirit act upon the paraform tablets, converting them into gas. Most of the conclusions derived from experiments with these three different forms of apparatus are the same. It is agreed that the gas is harmless to colours and metal and polished wood. The vapour acts best in a warm atmosphere. As for its action on bacteria, it compares favourably with any other disinfectant. In 1 per cent. solution formalin destroys non-spore-bearing bacteria in thirty to sixty minutes.
Many observers have decried formaldehyde on account of its professed lack of penetrating power. Professor Delépine, however, states[106] that it possesses "penetration powers probably greater than those of most other active gaseous disinfectants. _Bacillus coli_, _B. tuberculosis_, _B. pyocyaneus_, and _Staph. pyogenes aureus_ were killed in dry or moist state, even when protected by three layers of filter paper." In Professor Delépine's opinion, the vapours of phenol, izal, dry chlorine, and sulphurous acid have, under the same conditions, given inferior results.
We may now shortly summarise the foregoing facts respecting antiseptics and disinfection in the simplest terms possible to afford facility to the uninitiated in practical application:
_To disinfect a room_, seal up cracks and crevices, and burn at least one pound of roll sulphur for every 1,000 cubic feet of space.[107] Many authorities recommend four or five pounds of sulphur to the same space. Let the room remain sealed up for twenty-four hours.