It is an interesting fact, that while the direction in which the cilia propel fluids and particles is generally towards the interior of the organism, it is sometimes _reversed_; and, instead of beating the particles inwards, the cilia energetically beat them back, if they attempt to enter. Fatal results would ensue if this were not so. Our air-passages would no longer protect the lungs from particles of sand, coal-dust, and filings, flying about the atmosphere; on the contrary, the lashing hairs which cover the surface of these passages would catch up every particle, and drive it onwards into the lungs. Fortunately for us, the direction of the cilia is reversed, and they act as vigilant janitors, driving back all vagrant particles with a stern “No admittance--_even_ on business!” In vain does the whirlwind dash a column of dust in our faces--in vain does the air, darkened with coal-dust, impetuously rush up the nostrils: the air is allowed to pass on, but the dust is inexorably driven back. Were it not so, how could miners, millers, iron-workers, and all the modern Tubal Cains contrive to live in their loaded atmospheres? In a week, their lungs would be choked up. Perhaps, you will tell me that this _is_ the case: that manufacturers of iron and steel are very subject to consumption; and that there is a peculiar discoloration of the lungs which has often been observed in coal miners, examined after death. Not being a physician, and not intending to trouble you with medical questions, I must still place before you three considerations, which will show how untenable this notion is. First, although consumption may be frequent among the Sheffield workmen, the cause is not to be sought in their breathing filings, but in the sedentary and unwholesome confinement incidental to their occupation. Miners and coal-heavers are not troubled with consumption. Moreover, if the filings were the cause, all the artisans would suffer, when all breathe the same atmosphere. Secondly, while it is true that discoloured lungs have been observed in some miners, it has not been observed in all, or in many; whereas, it has been observed in men not miners, not exposed to any unusual amount of coal-dust. Thirdly, and most conclusively, experiment has shown that the coal-dust _cannot_ penetrate to the lungs. Claude Bernard, the brilliant experimenter, tied a bladder, containing a quantity of powdered charcoal, to the muzzle of a rabbit. Whenever the animal breathed, the powder within the bladder was seen to be agitated. Except during feeding time, the bladder was kept constantly on, so that the animal breathed only this dusty air. If the powder _could_ have escaped the vigilance of the cilia, and got into the lungs, this was a good occasion. But when the rabbit was killed and opened, many days afterwards, no powder whatever was found in the lungs, or bronchial tubes; several patches were collected about the nostrils and throat;