This section is from the "The Young Mother. Management of Children in Regard to Health" book, by William A. Alcott. Also available from Amazon: The Young Mother
I have already committed myself to the reader as favoring the use of soft flannel in cold weather, especially for children who are not yet able to run about freely in the open air. The advantages of an early use of this material, at least for under-clothes, are numerous. The following are a few of them.
1. Flannel, next to the skin, is a pleasant flesh brush; keeping up a gentle and equable irritation, and promoting perspiration and every other function which it is the office of the skin to perform, or assist in performing.
2. It guards the body against the cooling effects of evaporation, when in a state of profuse perspiration.
3. By preventing the heat of the body from escaping too rapidly, it keeps up a steadier temperature on the surface than any other known substance. The importance of the last consideration is greater, in a climate like our own, than elsewhere.
But there are limits to the use of this article of clothing. Whenever the temperature of the atmosphere is so great, even without artificial heat, that we no longer wish to retain the heat of the body by the clothing, then all flannel should be removed at once, and linen should be substituted; taking care to replace the flannel whenever the temperature of the atmosphere, as indicated by the thermometer, or by the child's feelings, may seem to require it.
It should also be kept clean. There is a very general mistake abroad on this subject. Many suppose that flannel can be worn longer without washing than other kinds of cloth. On the contrary, it should be changed oftener than cotton, or even linen, because it will absorb a great deal of fluid, especially the matter of perspiration, which, if long retained, is believed to ferment, and produce unhealthy, if not poisonous gases. For this reason, too, flannel for children's clothing should be white, that it may show dirt the more readily, and obtain the more frequent washing; although it is for this very reason—its liability to exhibit the least particles of dirt—that it is commonly rejected.
One caution more in regard to the use of flannel may be necessary. With some children, owing to a peculiarity of constitution, flannel will produce eruptions on the skin, which are very troublesome. Whenever this is the case, the flannel should be immediately laid aside; upon which the eruptions usually disappear.
If parents would take proper pains to get the lighter, softer kinds of flannel for this purpose, and be particular about its looseness and quantity, I should prefer, as I have already intimated, to have very young children, in our climate, wear this material the greater part of the year, excepting perhaps July and August.
My reasons for this course would be, first, that I like the stimulus of soft flannel on the skin, if changed sufficiently often, better than that of any other kind of clothing. Secondly, cotton is so liable to take fire, that its use in the nursery and among little children seems very hazardous. Thirdly, silk is not quite the appropriate material, as a general thing, besides being too expensive; and fourthly, linen is not warm enough, except in mid-summer.
Except, therefore, in July and August, and in cases of idiosyncracy, such as have just been alluded to, I would use flannel for the under-clothes of young children, throughout the year. But whenever they acquire sufficient strength to walk and run, and play much in the open air, I would gradually lay aside the use of all flannel, even in winter. Great attention, however, must be paid to the quantity. The parent who, guided by this rule, should keep on her child the same amount of flannel, and of the same thickness, from January to June 30th, and then, on the first of July, should suddenly exchange it for thin linen, in moderate quantity, might find trouble from it. It is better to make the changes more gradually; otherwise, whatever may be the material of the dress, the child will be likely to suffer.
 
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