How does "zu" change the meaning of the verb when it is inserted in between a word?
A specific example from a 18th century paper:
In der Tat gelang es nicht, auf dem Spectrum eines glühenden Platindrahtes oder des Drummondschen Lichtes die dunklen Natriumlinien durch die Leuchtgasflamme hervorzugerufen, in die Kochsalz gebracht war; aber es gelang mit der Flamme von wässrigem Alkohol, der Kochsalz enthielt.
In the dictionary, hervorrufen is listed, which means to "cause"? What is zu serving here?
Google Translation looks pretty good (as it makes scientific sense):
The absorption of the sodium vapor will be all the more perceptible, the lower its luminosity, that is, the lower its temperature. In fact, on the spectrum of a glowing platinum wire or the Drummond light, the dark sodium lines were not succeeded by the luminous gas flame into which salt was brought; but it succeeded with the flame of aqueous alcohol containing saline.