resulting habit of mind is apt to influence conduct in other spheres, where muscular power and nerve are of no avail—is apt to cause the daring of dangers which are not to be met by strength of limb or by skill. Nature, as externally presented in precipices, iceslopes, and crevasses, may be dared by one adequately endowed; but Nature, as internally presented in the form of physical constitution, may not be thus dared with impunity. Prompted by high motives, Tyndall tended too much to disregard the protests of his body. Over-application in Germany caused at one time absolute sleeplessness for, I think he told me, more than a week; and this, with kindred transgressions, brought on that insomnia by which his after-life was troubled, and by which his powers of work were diminished; for, as I have heard him say, a sound night's sleep was followed by marked exaltation of faculty. And then, in later life, came the daring which, by its results, brought his active career to a close. He conscientiously desired to fulfill an engagement to lecture at the Royal Institution, and was not to be deterred by fear of consequences. He gave the lecture, notwithstanding the protest which for days before his system had been making. The result was a serious illness, threatening, as he thought at one time, a fatal result; and, notwithstanding a year's furlough for the recovery of health, he was eventually obliged to resign his position. But for this defiance of Nature there might have been many more years of scientific exploration, pleasurable to himself and beneficial to others; and he might have escaped that invalid life which for a long time past he had to bear.
In his case, however, the penalties of invalid life had great mitigations—mitigations such as fall to the lot of but few. It is conceivable that the physical discomforts and mental weariness which ill-health brings may be almost compensated, if not even quite compensated, by the pleasurable emotions caused by unflagging attentions and sympathetic companionship. If this ever happens, it happened in his case. All who have known the household during these years of nursing are aware of the unmeasured kindness he has received without ceasing. I happen to have had special evidence of this devotion on the one side and gratitude on the other, which I do not think I am called upon to keep to myself, but rather to do the contrary. In a letter I received from him some half-dozen years ago, referring, among other things, to Mrs. Tyndall's self-sacrificing care of him, he wrote: "She has raised my ideal of the possibilities of human nature."