This kelp has been put to a fraudulent use, for Sir Robert Christison tells us that disease has been traced to an impure kind of salt, in which, when investigated, the hydriodate of soda was detected, resulting, he says, from an inferior salt obtained from kelp.[52] In all those districts which are intersected by salt marshes, there is almost a complete absence of miasmatic effluvia, though, as a natural consequence, the vegetation is not of that rank luxuriance which is invariably to be seen in other marsh lands; because, whenever the soil is in a state of moisture, it is always covered with all kinds of weeds and useless plants, which altogether stop the growth of those which are of utility to the agriculturist. In the case of salt marshes it is the reverse, and the neighbourhood is perfectly free from those endemic diseases which are prevalent in such localities as the fen-country, and other similar districts; for the atmosphere is pure, and the soil comparatively dry, and intermittent fever is unknown. CHAPTER VII. MEDICINAL AND DIETETIC PROPERTIES. Salt, except by the ignorant, is generally acknowledged to be a condiment, not only requisite as an adjunct to food, but also for the animal economy; this fact is not to be lost sight of, and therefore I lay much stress on it, and in the next chapter we shall see that physiologically it holds no mean position amongst those other substances which are found in the human body. There are a number of facts of physiological import, at which it is necessary to glance, and which are indissolubly connected with its medicinal and dietetic properties; and there are various others illustrative of the absolute necessity of salt, which are self-evident to those who think and observe, and which we will now proceed to lay before the reader.