An astronomer wants to observe a cloud of cold neutral (not ionized) hydrogen, far away from any stars. What would be an instrument that could help in this task?

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When the chemicals iron sulfide (fes) and hydrochloric acid (hcl) are combined, bubbles appear from the mixture. 1. does the appearance of bubbles indicate a physical or chemical change? 2. why do the bubbles indicate this change? 3. what property is this?

Who discovered a pattern to the elements in 1869?