9. Grease-traps placed inside a house--for instance, beneath the kitchen sink--are of very doubtful expediency, and if they cannot be placed outside, they had better not be used at all. In an unsewered city one of the first things to be considered in a sanitary inspection is the manner in which the sewage of the premises is disposed of. The question is, however, by no means superfluous in many sewered cities, for cesspools and vaults are to be found in most of them, and not only in yards, but beneath houses, and houses of the better class. A privy-vault or cesspool beneath a dwelling or near its cellar walls is always to be considered as very dangerous, for it is practically impossible to prevent the passage of gases from it into the interior of the house. A cesspit is a dangerous thing anywhere, even in the country; but in a city it is so dangerous that its existence should not be permitted. If the water-supply of a house is derived from a well, and there is reason to suspect that this may have been contaminated from a neighboring privy-vault, the first test to be applied to the water is that for the detection of chlorides. If none are present, the water is not polluted. If they are present, the quantity is to be noted, and a peck or two of common salt is then to be thrown into the suspected vault. If repeated {193} examinations of the water show a marked increase in the amount of chlorides present, it may be inferred that the contents of the privy pass to the well. The fact that the water of infected wells and springs is usually much liked and sought for is to a considerable extent due to the presence of these chlorides. Wanklyn recommends the addition of 50 grains of common salt per gallon to drinking water to render it palatable. Popularity of a certain well is therefore a reason for suspecting its purity. This subject may be dismissed with one caution. Taking the dwelling-houses of a city or town as they come, it will be found on examination that over half of them would be described by a competent inspector as being in a condition which might produce disease. It is therefore more than an even chance that in any case of disease some sanitary defect will be found about the premises quite irrespective of any direct causal connection with the case. Let the physician therefore be cautious in deciding as to such causal connection, and not conclude that because a case of diphtheria or typhoid fever and a leaky soil-pipe occur in the same house, therefore one is the cause of the other. Such cases occur in houses whose sewerage is perfect and in houses which have no sewerage, and it is folly to attribute them exclusively or mainly to sewer gases.