One Myrtle Proves Nothing: Repeated Comparative Experiments and the Growing Awareness of the Difficulty of Conducting Conclusive Experiments

This chapter focuses on physicists from across Europe who, between the mid-1740s and the mid-1780s, investigated whether electricity promoted plant growth. They considered comparative experiments to be a prerequisite for making reliable causal inferences, and thus compared the growth of electrified and non-electrified plants. In doing so, they were careful to keep the two experimental setups stable to rule out the possibility that growth differences were caused by factors other than electrification. A closer look at the experimental reports of these physicists reveals sophisticated discussions of control strategies. They pointed out several possible sources of error and suggested strategies for avoiding them, such as repeating an experiment many times to eliminate chance effects. But the physicists’ methodological views also differed on crucial issues, such as how many plants an experiment should include, or how often it should be repeated to ensure a reliable conclusion. By the late 1780s, the question of electricity’s effect on plant growth was far from resolved. Rather, the controversy became emblematic of the difficulty of conducting well-controlled, conclusive experiments.