=Fatal Experiment of Richman.=--There can be no doubt that experiments of this kind, made with the electricity of a thundercloud, were extremely dangerous; and this was soon proved by a fatal accident. Professor Richman, of St. Petersburgh, had erected on the roof of his house a pointed iron rod, the lower end of which passed into a glass vessel, intended, as we are informed, to measure the strength of the charge which he expected to receive from the clouds. On the sixth of August, 1753, observing the approach of a thunderstorm, he hastened to his apparatus; and as he stood near it, with his head bent down, to watch the effect, a flash of lightning passed through his body and killed him on the spot. This catastrophe served to fix public attention on the danger of such experiments, and gave occasion to the saying of Voltaire: “There are some great lords whom we should always approach with extreme precaution, and lightning is one of them.”[1] From this time the practice of making experiments directly with the lightning of the clouds seems to have been, by common consent, abandoned. =Immediate Cause of Lightning.=--And now, having set before you some of the most memorable experiments by which the identity of lightning and electricity has been demonstrated, I will try to give you a clear conception regarding the immediate cause of lightning, so far as the subject is understood at the present day by scientific men. You know that there are two kinds of electricity, which are called _positive_ and _negative_; and that each of them repels electricity of the same kind as itself, while it attracts electricity of the opposite kind. Now, every thundercloud is charged with electricity of one kind or the other, positive or negative; and, as it hovers over the earth, it develops, by what is called _induction_, or influence, electricity of the opposite kind in that part of the earth which is immediately under it. Thus we have two bodies--the cloud and the earth--charged with opposite kinds of electricity, and separated by a stratum of the atmosphere. The two opposite electricities powerfully attract each other; but for a time they are prevented from rushing together by the intervening stratum of air, which is a non-conductor of electricity, and acts as a barrier between them. As the electricity, however, continues to accumulate, the attraction becomes stronger and stronger, until at length it is able to overcome the resistance of this barrier; a violent disruptive discharge then takes place between the cloud and the earth, and the flash of lightning is the consequence of the discharge. [Illustration: THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING.]