With respect to other familiar and more or less poisonous substances used to preserve foods, defense of their harmlessness is far more difficult. Formaldehyde, salicylic acid, sulphurous acid, and sulphite are compounds definitely poisonous in relatively small amounts, their injurious action in minute successive doses in animal experiments is quite marked, and their use in human food products practically without justification. Boric acid and borax are perhaps on a slightly different footing, but are never present in natural foods, and there is no good evidence that their long-continued ingestion in small doses is without injurious effect. It must not be forgotten that all such substances owe their preservative or antiseptic power to the poisonous effect they have upon bacterial protoplasm. It is fair to assume that, in general, bacterial protoplasm is no more easily injured than human protoplasm, and this raises at once the propriety of bringing into repeated contact with human tissues substances likely to produce injury even if such injury is slight and recovery from it is ordinarily easy. In every case the burden of proof should be properly placed on those who advocate the addition of bacterial-restraining substances to food intended for human consumption. It is for them to show that substances powerful enough to hold in check the development of bacteria are yet unable to interfere seriously with the life-processes of the cells of the human body. When this view of the situation is taken, not only the chemical substances mentioned previously fall under some suspicion, but also certain household preservatives long sanctioned by custom. Spices such as cinnamon, oil of cloves, and the like are, so far as we know, as likely to have an injurious physiological effect when taken in small recurring quantities as are some of the "chemical" preservatives whose use is debarred by law. The chemicals deposited by wood smoke in meat are of a particularly objectionable nature, and their continuous ingestion may quite conceivably lead to serious injury. One fact persistently comes to the front in any comprehensive study of the food-preservative question, namely, the need of further experiment and observation. We do not at present know what effect is produced in human beings of different ages and varying degrees of strength by the _long-continued_ consumption of food preserved with particular chemicals.