A large and important series of hypnotics are classed in this group. Chloral, the point of departure of the others, has been in use for a number of years, and in the attempt to produce a similar body free from the disagreeable taste and from some of the more depressing cardiac action, a host of alcohol halogens has been made and introduced into therapeutics.

The introduction of chlorine into the hydrocarbon series produces a more marked change in the action than has been noted for many of the alcoholic derivatives already discussed.

Ordinary marsh gas, CH4, has a very slight narcotic action. CH3Cl is stronger, CH1C12 still stronger, CHCl3 (chloroform) very active, and CC14 very highly toxic. A gradual increase in toxicity has been observed with the addition of chlorine atoms. In the same manner aldehyde is a mild narcotic, trichloraldehyde much more powerful. A large number of these compounds have been employed in anesthesia; these have already been discussed.

For the most part the newer chloral substitutes are known to break down into chloral in the metabolism of the body. On general principles, therefore, it is difficult to see wherein they are to offer any marked advantages over the parent body itself. If, however, the compound with which it is combined is so split off in the body and is then so unmodified as to be active and to be able to overcome the untoward effects of the chloral or its reduction compounds, a useful hypnotic must be the result. Up to the present time it does not appear from other sources than those with a commercial bias that this very desirable combination has been found. There is no reason to believe that it may not be accomplished, but nearly all the theoretically possible combinations have been made and tried. A number of the compounds now manufactured are useful nevertheless, and will be here considered.