When did the old term experimental reagent become experimental subject? I've read a paper on an experiment called "The Feeling of Being Stared at", which investigates a concept which on Wikipedia is called the psychic staring effect. It was first investigated by psychologist Edward B. Titchener in 1898.

The paper consistently refers to the experimental subjects as "reagents". 


  "In some of the experiments, the distance between the experimenter and
  reagent was varied for the purpose of..."
  Page 3 of 6


I'm unfamiliar with this term to refer to subjects, and I assume it's because this is an old paper from 1913. My guess would be that the word means something or something that reacts, and subjects are subjected to stimuli. Is anyone familiar with this term being used generally in experiments around this time period or even today? And if so, when it fell out of fashion and replaced with what we use now: "subject".
Psychological theory has often metaphorically used terminology from the technology or science of its time.

Freud explains certain aspects of the psyche in terms of the steam engine, and psychologists of the late 20th century think of the brain in terms of computer science.

In your case Titchener and other psychologists of his time tried to grasp psychology in terms of chemistry: "a reagent is a substance which, because it has a capacity for certain reactions, is used to detect, examine, or measure other substances" (Schultz, 2013, p. 88). Historians of psychology assess this term as evidence for the mechanistic view inherent in the structuralist approach to psychology. For Titchener, the human proband is a recording instrument (Schultz, 2013, p. 88).

As psychology as a science has shifted to a new paradigm in the early 20th century, the term reagent slowly fell out of use. The latest use in a psychological context that I'm aware of is in the late 1920s.



Sources:


Schultz, D. (2013). A History of Modern Psychology (2nd ed). New York: Academic Press.