December, 1871, Professor Barker delivered a lecture before the American Institute in New York, upon the "Correlation of Vital and Physical Forces," which attracted very general attention. The lecture was an attempt to show that, besides the ordinary psychological definition of mind, another and a purely physiological one might be found, which represented mind as solely the product of brain-action, and, as such, entirely capable of being correlated with physical forces. In 1872 he was Vice-President of the American Association at its Indianapolis meeting.
Having made the branch of toxicology the subject of special study, Professor Barker was engaged quite generally in the investigation of cases of criminal poisoning. Perhaps the most important of these cases was the celebrated one in which Lydia Sherman was tried in New Haven, in April, 1872, for poisoning her husband with arsenic. Because the Wharton case, tried just before, had apparently left upon the public mind the impression that chemical analysis in such cases was unreliable, and hence had given the criminally disposed some reason to believe that their might commit murder by poison with impunity, especial care was taken by Dr. Barker to present the chemical and physiological evidence in the Sherman case in a fully conclusive form. To the thoroughness of this preparation, and the completeness of the chemical evidence, the conviction of the prisoner was largely due. The chemical evidence in this trial, after correction by him, was inserted in full, as a typical case, in the subsequent edition of Wharton and Stillé's "Medical Jurisprudence."
In February, 1873, he was strongly urged to accept the chair of Physics in the University of Pennsylvania, Philadelphia. After due consideration and consultation the offer was accepted, and he removed from New Haven to Philadelphia in April. The trustees having placed a generous sum of money at his disposal, for the purpose of providing the apparatus necessary for illustrating the science in a proper manner, Professor Barker left in July for Europe, in order to personally inspect the instruments he was about to purchase. The result has justified this step. The collection of physical apparatus in the university cabinet is certainly unsurpassed in this country, and in some directions it is absolutely unique in the world.
In the fall of 1876 Professor Barker had the distinguished honor of being elected a member of the National Academy of Sciences. In the summer of 1878, on invitation of Professor Henry Draper, he accompanied the Draper Eclipse Expedition to Rawlins, Wyoming, where he studied the total solar eclipse of July 29th, as spectroscopic observer. The most important fact obtained by him was the confirmation of Janssen's observation of 1871, that the coronal spectrum contained the dark solar lines of Fraunhofer. After the eclipse he accompanied his friend Thomas A. Edison on a trip to California and the Yosemite. He stopped on his return to attend the meeting of the
